National Instruments NI USB-621x User Manual

Download

DAQ M Series

NI USB-621x User Manual

Bus-Powered M Series USB Devices

NI USB-621x User Manual

August 2006 371931A-01

Support

Worldwide Technical Support and Product Information

ni.com

National Instruments Corporate Headquarters

11500 North Mopac Expressway Austin, Texas 78759-3504 USA Tel: 512 683 0100

Worldwide Offices

Australia 1800 300 800, Austria 43 0 662 45 79 90 0, Belgium 32 0 2 757 00 20, Brazil 55 11 3262 3599, Canada 800 433 3488, China 86 21 6555 7838, Czech Republic 420 224 235 774, Denmark 45 45 76 26 00, Finland 385 0 9 725 725 11, France 33 0 1 48 14 24 24, Germany 49 0 89 741 31 30, India 91 80 41190000, Israel 972 0 3 6393737, Italy 39 02 413091, Japan 81 3 5472 2970, Korea 82 02 3451 3400, Lebanon 961 0 1 33 28 28, Malaysia 1800 887710, Mexico 01 800 010 0793, Netherlands 31 0 348 433 466, New Zealand 0800 553 322, Norway 47 0 66 90 76 60, Poland 48 22 3390150, Portugal 351 210 311 210, Russia 7 095 783 68 51, Singapore 1800 226 5886, Slovenia 386 3 425 4200, South Africa 27 0 11 805 8197, Spain 34 91 640 0085, Sweden 46 0 8 587 895 00, Switzerland 41 56 200 51 51, Taiwan 886 02 2377 2222, Thailand 662 278 6777, United Kingdom 44 0 1635 523545

For further support information, refer to the Technical Support and Professional Services appendix. To comment on National Instruments documentation, refer to the National Instruments Web site at the info code

feedback.

ni.com/info and enter

Important Information

Warranty

The USB-6210, USB-6211, USB-6215, and USB-6218 devices are warranted against defects in materials and workmanship for a period of three years from the date of shipment, as evidenced by receipts or other documentation. National Instruments will, at its option, repair or replace equipment that proves to be defective during the warranty period. This warranty includes parts and labor.

The media on which you receive National Instruments software are warranted not to fail to execute programming instructions, due to defects in materials and workmanship, for a period of 90 days from date of shipment, as evidenced by receipts or other documentation. National Instruments will, at its option, repair or replace software media that do not execute programming instruc tions if National Instruments receives notice of such defects during the warranty period. National Instruments does not warrant that the operation of the software shall be uninterrupted or error free.

A Return Material Authorization (RMA) number must be obtained from the factory and clearly marked on the outside of the package before any equipment will be accepted for warranty work. National Instruments will pay the shipping costs of returning to the owner parts which are covered by warranty.

National Instruments believes that the information in this document is accurate. The document has been carefully reviewed for technical accuracy. In the event that technical or typographical errors exist, National Instruments reserves the right to make changes to subsequent editions of this document without prior notice to holders of this edition. The reader should consult National Instruments if errors are suspected. In no event shall National Instruments be liable for any damages arising out of or related to this document or the information contained in it.

XCEPT AS SPECIFIED HEREIN, NATIONAL INSTRUMENTS MAKES NO WARRANTIES, EXPRESS OR IMPLIED, AND SPECIFICALLY DISCLAIMS ANY WARRANTY OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. CUSTOMER’S RIGHT TO RECOVER DAMAGES CAUSED BY FAULT OR NEGLIGENCE ON THE PART OF NATIONAL

NSTRUMENTS SHALL BE LIMITED TO THE AMOUNT THERETOFORE PAID BY THE CUSTOMER. NATIONAL INSTRUMENTS WILL NOT BE LIABLE FOR DAMAGES RESULTING

FROM LOSS OF DATA, PROFITS, USE OF PRODUCTS, OR INCIDENTAL OR CONSEQUENTIAL DAMAGES, EVEN IF ADVISED OF THE POSSIBILITY THEREOF. This limitation of

the liability of National Instruments will apply regardless of the form of action, whether in contract or tort, including negligence. Any action against National Instruments must be brought within one year after the cause of action accrues. National Instruments shall not be liable for any delay in performance due to causes beyond its reasonable control. The warranty provided herein does not cover damages, defects, malfunctions, or service failures caused by owner’s failure to follow the National Instruments installation, operation, or maintenance instructions; owner’s modification of the product; owner’s abuse, misuse, or negligent acts; and power failure or surges, fire, flood, accident, actions of third parties, or other events outside reasonable control.

Copyright

Under the copyright laws, this publication may not be reproduced or transmitted in any form, electronic or mechanical, including photocopying, recording, storing in an information retrieval system, or translating, in whole or in part, without the prior written consent of National Instruments Corporation.

National Instruments respects the intellectual property of others, and we ask our users to do the same. NI software is protected by copyright and other intellectual property laws. Where NI software may be used to reproduce software or other materials belonging to others, you may use NI software only to reproduce materials that you may reproduce in accordance with the terms of any applicable license or other legal restriction.

Trademarks

National Instruments, NI, ni.com, and LabVIEW are trademarks of National Instruments Corporation. Refer to the Terms of Use section on

ni.com/legal for more information about National Instruments trademarks.

is the registered trademark of Apple Computer, Inc. Other product and company names mentioned herein are trademarks or trade names

FireWire of their respective companies.

Members of the National Instruments Alliance Partner Program are business entities independent from National Instruments and have no agency, partnership, or joint-venture relationship with National Instruments.

Patents

For patents covering National Instruments products, refer to the appropriate location: Help»Patents in your software, the patents.txt file on your CD, or

ni.com/patents.

WARNING REGARDING USE OF NATIONAL INSTRUMENTS PRODUCTS

(1) NATIONAL INSTRUMENTS PRODUCTS ARE NOT DESIGNED WITH COMPONENTS AND TESTING FOR A LEVEL OF RELIABILITY SUITABLE FOR USE IN OR IN CONNECTION WITH SURGICAL IMPLANTS OR AS CRITICAL COMPONENTS IN ANY LIFE SUPPORT SYSTEMS WHOSE FAILURE TO PERFORM CAN REASONABLY BE EXPECTED TO CAUSE SIGNIFICANT INJURY TO A HUMAN.

(2) IN ANY APPLICATION, INCLUDING THE ABOVE, RELIABILITY OF OPERATION OF THE SOFTWARE PRODUCTS CAN BE IMPAIRED BY ADVERSE FACTORS, INCLUDING BUT NOT LIMITED TO FLUCTUATIONS IN ELECTRICAL POWER SUPPLY, COMPUTER HARDWARE MALFUNCTIONS, COMPUTER OPERATING SYSTEM SOFTWARE FITNESS, FITNESS OF COMPILERS AND DEVELOPMENT SOFTWARE USED TO DEVELOP AN APPLICATION, INSTALLATION ERRORS, SOFTWARE AND HARDWARE COMPATIBILITY PROBLEMS, MALFUNCTIONS OR FAILURES OF ELECTRONIC MONITORING OR CONTROL DEVICES, TRANSIENT FAILURES OF ELECTRONIC SYSTEMS (HARDWARE AND/OR SOFTWARE), UNANTICIPATED USES OR MISUSES, OR ERRORS ON THE PART OF THE USER OR APPLICATIONS DESIGNER (ADVERSE FACTORS SUCH AS THESE ARE HEREAFTER COLLECTIVELY TERMED “SYSTEM FAILURES”). ANY APPLICATION WHERE A SYSTEM FAILURE WOULD CREATE A RISK OF HARM TO PROPERTY OR PERSONS (INCLUDING THE RISK OF BODILY INJURY AND DEATH) SHOULD NOT BE RELIANT SOLELY UPON ONE FORM OF ELECTRONIC SYSTEM DUE TO THE RISK OF SYSTEM FAILURE. TO AVOID DAMAGE, INJURY, OR DEATH, THE USER OR APPLICATION DESIGNER MUST TAKE REASONABLY PRUDENT STEPS TO PROTECT AGAINST SYSTEM FAILURES, INCLUDING BUT NOT LIMITED TO BACK-UP OR SHUT DOWN MECHANISMS. BECAUSE EACH END-USER SYSTEM IS CUSTOMIZED AND DIFFERS FROM NATIONAL INSTRUMENTS' TESTING PLATFORMS AND BECAUSE A USER OR APPLICATION DESIGNER MAY USE NATIONAL INSTRUMENTS PRODUCTS IN COMBINATION WITH OTHER PRODUCTS IN A MANNER NOT EVALUATED OR CONTEMPLATED BY NATIONAL INSTRUMENTS, THE USER OR APPLICATION DESIGNER IS ULTIMATELY RESPONSIBLE FOR VERIFYING AND VALIDATING THE SUITABILITY OF NATIONAL INSTRUMENTS PRODUCTS WHENEVER

NATIONAL INSTRUMENTS PRODUCTS ARE INCORPORATED IN A SYSTEM OR APPLICATION, INCLUDING, WITHOUT LIMITATION, THE APPROPRIATE DESIGN, PROCESS AND SAFETY LEVEL OF SUCH SYSTEM OR APPLICATION.

About This Manual

Conventions ...................................................................................................................xiii

Related Documentation..................................................................................................xiv

NI-DAQmx for Windows................................................................................xiv

LabVIEW ........................................................................................................xiv

LabWindows™/CVI™....................................................................................xv

Measurement Studio........................................................................................xv

ANSI C without NI Application Software ......................................................xv

.NET Languages without NI Application Software ........................................xvi

Device Documentation and Specifications......................................................xvi

Training Courses .............................................................................................xvi

Technical Support on the Web ........................................................................xvi

Chapter 1 Getting Started

Installing NI-DAQmx ....................................................................................................1-2

Installing Other Software...............................................................................................1-2

Installing the Hardware..................................................................................................1-3

Device Pinouts ...............................................................................................................1-3

Device Specifications ....................................................................................................1-3

Device Accessories ........................................................................................................1-3

Chapter 2 DAQ System Overview

DAQ Hardware ..............................................................................................................2-1

DAQ-STC2......................................................................................................2-2

Calibration Circuitry........................................................................................2-2

Signal Conditioning .......................................................................................................2-3

Sensors and Transducers .................................................................................2-3

Programming Devices in Software ................................................................................2-4

Chapter 3 Connector Information

I/O Connector Signal Descriptions ................................................................................3-1

+5 V Power ....................................................................................................................3-2

+5 V Power as an Output ................................................................................3-2

+5 V Power as an Input ...................................................................................3-3

Contents

Chapter 4 Analog Input

Analog Input Circuitry .................................................................................................. 4-1

Analog Input Range....................................................................................................... 4-2

Analog Input Ground-Reference Settings ..................................................................... 4-3

Multichannel Scanning Considerations ......................................................................... 4-6

Analog Input Data Acquisition Methods....................................................................... 4-9

Analog Input Digital Triggering.................................................................................... 4-10

Field Wiring Considerations..........................................................................................4-11

Analog Input Timing Signals ........................................................................................ 4-11

Configuring AI Ground-Reference Settings in Software................................ 4-5

Use Low Impedance Sources.......................................................................... 4-6

Carefully Choose the Channel Scanning Order ..............................................4-7

Avoid Switching from a Large to a Small Input Range ................... 4-7

Insert Grounded Channel between Signal Channels ........................ 4-7

Minimize Voltage Step between Adjacent Channels ....................... 4-8

Avoid Scanning Faster Than Necessary ......................................................... 4-8

Example 1 ......................................................................................... 4-8

Example 2 ......................................................................................... 4-9

Software-Timed Acquisitions ......................................................................... 4-9

Hardware-Timed Acquisitions........................................................................ 4-9

Buffered ............................................................................................ 4-10

Non-Buffered.................................................................................... 4-10

AI Sample Clock Signal.................................................................................. 4-14

Using an Internal Source .................................................................. 4-15

Using an External Source ................................................................. 4-15

Routing AI Sample Clock Signal to an Output Terminal................. 4-15

Other Timing Requirements ............................................................. 4-15

AI Sample Clock Timebase Signal ................................................................. 4-16

AI Convert Clock Signal................................................................................. 4-16

Using an Internal Source .................................................................. 4-17

Using an External Source ................................................................. 4-17

Routing AI Convert Clock Signal to an Output Terminal ................ 4-17

Using a Delay from Sample Clock to Convert Clock ...................... 4-17

Other Timing Requirements ............................................................. 4-18

AI Convert Clock Timebase Signal ................................................................ 4-20

AI Hold Complete Event Signal ..................................................................... 4-21

AI Start Trigger Signal.................................................................................... 4-21

Using a Digital Source...................................................................... 4-21

Routing AI Start Trigger to an Output Terminal .............................. 4-21

AI Reference Trigger Signal ........................................................................... 4-22

Using a Digital Source...................................................................... 4-23

Routing AI Reference Trigger Signal to an Output Terminal .......... 4-23

NI USB-621x User Manual vi ni.com

AI Pause Trigger Signal ..................................................................................4-23

Using a Digital Source ......................................................................4-23

Getting Started with AI Applications in Software.........................................................4-24

Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

Connecting Floating Signal Sources..............................................................................5-3

What Are Floating Signal Sources? ................................................................5-3

When to Use Differential Connections with Floating Signal Sources ............5-3

When to Use Referenced Single-Ended (RSE) Connections with

Floating Signal Sources................................................................................5-3

When to Use Non-Referenced Single-Ended (NRSE) Connections

with Floating Signal Sources........................................................................5-4

Using Differential Connections for Floating Signal Sources ..........................5-5

Using Non-Referenced Single-Ended (NRSE) Connections for

Floating Signal Sources................................................................................5-8

Using Referenced Single-Ended (RSE) Connections for Floating

Signal Sources ..............................................................................................5-9

Connecting Ground-Referenced Signal Sources ...........................................................5-9

What Are Ground-Referenced Signal Sources? ..............................................5-9

When to Use Differential Connections with Ground-Referenced

Signal Sources ..............................................................................................5-10

When to Use Non-Referenced Single-Ended (NRSE) Connections with

Ground-Referenced Signal Sources..............................................................5-10

When to Use Referenced Single-Ended (RSE) Connections with

Ground-Referenced Signal Sources..............................................................5-11

Using Differential Connections for Ground-Referenced Signal Sources........5-12

Using Non-Referenced Single-Ended (NRSE) Connections for

Ground-Referenced Signal Sources..............................................................5-13

Contents

Chapter 6 Connecting AI Signals on the USB-6215/6218 Devices

Differential Measurements ............................................................................................6-1

Differential Pairs............................................................................................................6-1

Referenced Single-Ended (RSE) Measurements ...........................................................6-3

Non-Referenced Single-Ended (NRSE) Measurements ................................................6-4

Contents

Chapter 7 Analog Output

Analog Output Circuitry................................................................................................ 7-1

AO Range ...................................................................................................................... 7-2

Minimizing Glitches on the Output Signal.................................................................... 7-2

Analog Output Data Generation Methods ..................................................................... 7-2

Software-Timed Generations .......................................................................... 7-2

Hardware-Timed Generations......................................................................... 7-2

Analog Output Digital Triggering................................................................................. 7-4

Connecting Analog Output Signals ............................................................................... 7-4

Analog Output Timing Signals...................................................................................... 7-5

AO Start Trigger Signal .................................................................................. 7-5

AO Pause Trigger Signal ................................................................................ 7-6

AO Sample Clock Signal ................................................................................ 7-7

AO Sample Clock Timebase Signal................................................................ 7-8

Getting Started with AO Applications in Software....................................................... 7-9

Using a Digital Source...................................................................... 7-5

Routing AO Start Trigger Signal to an Output Terminal ................. 7-6

Using a Digital Source...................................................................... 7-6

Using an Internal Source .................................................................. 7-7

Using an External Source ................................................................. 7-7

Routing AO Sample Clock Signal to an Output Terminal ............... 7-7

Other Timing Requirements ............................................................. 7-7

Chapter 8 Digital I/O

Static DIO......................................................................................................................8-2

I/O Protection ................................................................................................................ 8-2

Increasing Current Drive ............................................................................................... 8-2

Connecting Digital I/O Signals ..................................................................................... 8-3

Getting Started with DIO Applications in Software...................................................... 8-4

Chapter 9 Counters

Counter Input Applications ........................................................................................... 9-2

Counting Edges ............................................................................................... 9-2

Single Point (On-Demand) Edge Counting ...................................... 9-2

Buffered (Sample Clock) Edge Counting......................................... 9-3

Non-Cumulative Buffered Edge Counting ....................................... 9-4

Controlling the Direction of Counting.............................................. 9-4

NI USB-621x User Manual viii ni.com

Contents

Pulse-Width Measurement ..............................................................................9-5

Single Pulse-Width Measurement.....................................................9-5

Buffered Pulse-Width Measurement.................................................9-5

Period Measurement........................................................................................9-6

Single Period Measurement ..............................................................9-7

Buffered Period Measurement ..........................................................9-7

Semi-Period Measurement ..............................................................................9-9

Single Semi-Period Measurement.....................................................9-9

Buffered Semi-Period Measurement.................................................9-9

Frequency Measurement .................................................................................9-10

Method 1—Measure Low Frequency with One Counter..................9-10

Method 1b—Measure Low Frequency with One Counter

(Averaged)......................................................................................9-11

Method 2—Measure High Frequency with Two Counters...............9-11

Method 3—Measure Large Range of Frequencies Using

Two Counters .................................................................................9-12

Choosing a Method for Measuring Frequency .................................9-13

Position Measurement .....................................................................................9-15

Measurements Using Quadrature Encoders......................................9-15

Measurements Using Two Pulse Encoders.......................................9-17

Two-Signal Edge-Separation Measurement....................................................9-18

Single Two-Signal Edge-Separation Measurement ..........................9-18

Buffered Two-Signal Edge-Separation Measurement ......................9-19

Counter Output Applications .........................................................................................9-20

Simple Pulse Generation .................................................................................9-20

Single Pulse Generation ....................................................................9-20

Single Pulse Generation with Start Trigger ......................................9-20

Retriggerable Single Pulse Generation .............................................9-21

Pulse Train Generation ....................................................................................9-22

Continuous Pulse Train Generation ..................................................9-22

Frequency Generation .....................................................................................9-23

Using the Frequency Generator ........................................................9-23

Frequency Division .........................................................................................9-24

Pulse Generation for ETS................................................................................9-24

Counter Timing Signals.................................................................................................9-25

Counter n Source Signal..................................................................................9-26

Routing a Signal to Counter n Source...............................................9-26

Routing Counter n Source to an Output Terminal ............................9-26

Counter n Gate Signal .....................................................................................9-27

Routing a Signal to Counter n Gate ..................................................9-27

Routing Counter n Gate to an Output Terminal................................9-27

Counter n Aux Signal ......................................................................................9-27

Routing a Signal to Counter n Aux ...................................................9-27

Contents

Counter n A, Counter n B, and Counter n Z Signals ...................................... 9-28

Routing Signals to A, B, and Z Counter Inputs................................ 9-28

Counter n Up_Down Signal............................................................................ 9-28

Counter n HW Arm Signal.............................................................................. 9-28

Routing Signals to Counter n HW Arm Input.................................. 9-28

Counter n Internal Output and Counter n TC Signals..................................... 9-29

Routing Counter n Internal Output to an Output Terminal .............. 9-29

Frequency Output Signal ................................................................................ 9-29

Routing Frequency Output to a Terminal......................................... 9-29

Default Counter/Timer Pinouts ..................................................................................... 9-29

Counter Triggering ........................................................................................................ 9-31

Arm Start Trigger............................................................................................ 9-31

Start Trigger .................................................................................................... 9-31

Pause Trigger .................................................................................................. 9-31

Other Counter Features.................................................................................................. 9-32

Sample Clock .................................................................................................. 9-32

Cascading Counters......................................................................................... 9-33

Counter Filters................................................................................................. 9-33

Prescaling ........................................................................................................ 9-34

Duplicate Count Prevention ............................................................................ 9-35

Example Application That Works Correctly

(No Duplicate Counting) ............................................................... 9-36

Example Application That Works Incorrectly

(Duplicate Counting) ..................................................................... 9-37

Example Application That Prevents Duplicate Count...................... 9-37

Enabling Duplicate Count Prevention in NI-DAQmx...................... 9-38

Chapter 10 PFI

Using PFI Terminals as Timing Input Signals .............................................................. 10-2

Exporting Timing Output Signals Using PFI Terminals............................................... 10-3

Using PFI Terminals as Static Digital I/Os ................................................................... 10-3

Connecting PFI Input Signals........................................................................................ 10-3

PFI Filters ...................................................................................................................... 10-4

I/O Protection ................................................................................................................ 10-6

Programmable Power-Up States.................................................................................... 10-6

NI USB-621x User Manual x ni.com

Chapter 11 Isolation and Digital Isolators

Digital Isolation .............................................................................................................11-2

Benefits of an Isolated DAQ Device .............................................................................11-2

Reducing Common-Mode Noise ...................................................................................11-2

Creating an AC Return Path............................................................................11-3

Isolated Systems................................................................................11-3

Non-Isolated Systems .......................................................................11-3

Chapter 12 Digital Routing and Clock Generation

80 MHz Timebase..........................................................................................................12-1

20 MHz Timebase..........................................................................................................12-1

100 kHz Timebase .........................................................................................................12-2

Chapter 13 Bus Interface

USB Signal Streams.......................................................................................................13-1

Data Transfer Methods ..................................................................................................13-1

USB Signal Stream..........................................................................................13-1

Programmed I/O ..............................................................................................13-2

Changing Data Transfer Methods ...................................................................13-2

Contents

Chapter 14 Triggering

Triggering with a Digital Source ...................................................................................14-1

Appendix A Device-Specific Information

USB-6210 ......................................................................................................................A-1

USB-6211/6215 .............................................................................................................A-4

USB 6218.......................................................................................................................A-7

Contents

Appendix B Troubleshooting

Appendix C Technical Support and Professional Services

Glossary

Index

Device Pinouts

Figure A-1. USB-6210 Pinout .................................................................................. A-2

Figure A-2. USB-6211/6215 Pinout ......................................................................... A-5

Figure A-3. USB 6218 Pinout .................................................................................. A-8

NI USB-621x User Manual xii ni.com

About This Manual

The NI 621x User Manual contains information about using the National Instruments USB-621x data acquisition (DAQ) devices with NI-DAQmx 8.3 and later. NI 621x devices feature up to 32 analog input (AI) channels, up to two analog output (AO) channels, up to eight lines of digital input (DI), up to eight lines of digital output (DO), and two counters.

Conventions

The following conventions are used in this manual:

<> Angle brackets that contain numbers separated by an ellipsis represent

a range of values associated with a bit or signal name—for example, AO <3..0>.

» The » symbol leads you through nested menu items and dialog box options

to a final action. The sequence File»Page Setup»Options directs you to pull down the File menu, select the Page Setup item, and select Options from the last dialog box.

This icon denotes a note, which alerts you to important information.

This icon denotes a caution, which advises you of precautions to take to avoid injury, data loss, or a system crash. When this symbol is marked on a product, refer to the Read Me First: Safety and Radio-Frequency Interference document which can be found at information about precautions to take.

bold Bold text denotes items that you must select or click in the software, such

as menu items and dialog box options. Bold text also denotes parameter names.

italic Italic text denotes variables, emphasis, a cross-reference, or an introduction

to a key concept. Italic text also denotes text that is a placeholder for a word or value that you must supply.

monospace Text in this font denotes text or characters that you should enter from the

keyboard, sections of code, programming examples, and syntax examples. This font is also used for the proper names of disk drives, paths, directories, programs, subprograms, subroutines, device names, functions, operations, variables, filenames, and extensions.

ni.com/manuals, for

About This Manual

Figure 1-1. USB-6210/6211

Chapter 1 Getting Started

Figure 1-2. USB-6215/6218

NI 621x devices feature up to 32 analog input (AI) channels, up to two analog output (AO) channels, 8 lines of digital input (DI), 8 lines of digital output (DO), and two counters. If you have not already installed your device, refer to the NI-DAQmx for USB Devices Getting Started Guide. For specifications, refer to the NI 621x Specifications document on

ni.com/manuals.

Before installing your DAQ device, you must install the software you plan to use with the device.

Installing NI-DAQmx

The NI-DAQmx for USB Devices Getting Started Guide, which you can download at instructions for installing software and hardware, configuring channels and tasks, and getting started developing an application.

ni.com/manuals, offers NI-DAQmx users step-by-step

Installing Other Software

If you are using other software, refer to the installation instructions that accompany your software.

NI USB-621x User Manual 1-2 ni.com

Installing the Hardware

The NI-DAQmx for USB Devices Getting Started Guide contains non-software-specific information about how to install USB devices.

Device Pinouts

Refer to Appendix A, Device-Specific Information, for NI 621x device pinouts.

Device Specifications

Refer to the NI 621x Specifications, available on the NI-DAQ Device Document Browser or about NI 621x devices.

Device Accessories

NI offers a variety of accessories to use with your DAQ device. Refer to Appendix A, Device-Specific Information, or information.

Chapter 1 Getting Started

ni.com/manuals, for more detailed information

ni.com for more

DAQ System Overview

Figure 2-1 shows a typical DAQ system, which includes sensors, transducers, signal conditioning devices, cables that connect the various devices to the accessories, the M Series device, programming software, and PC. The following sections cover the components of a typical DAQ system.

DAQ Hardware

DAQ

Hardware

Figure 2-1. Components of a Typical DAQ System

DAQ hardware digitizes signals, performs D/A conversions to generate analog output signals, and measures and controls digital I/O signals. Figure 2-2 features components common to all USB M Series devices.

DAQ

Software

Personal Computer

or Laptop

Chapter 2 DAQ System Overview

Analog Input

Analog Output

Digital I/O

I/O Connector

Counters

PFI

DAQ-STC2

The DAQ-STC2 implements a high-performance digital engine for M Series data acquisition hardware. Some key features of this engine include the following:

• Flexible AI and AO sample and convert timing

• Many triggering modes

• Independent AI, AO, and CTR FIFOs

• Generation and routing of internal and external timing signals

• Two flexible 32-bit counter/timer modules with hardware gating

• Static DI and static DO signals

• USB Hi-Speed 2.0 interface

• Up to four USB Signal Streams for acquisition and generation

and Clock

Generation

functions

Digital

Routing

Isolation

Barrier

(USB-6215

and USB-6218

devices only)

Digital

Isolators

Bus

Interface

Figure 2-2. USB-621x Block Diagram

Bus

Calibration Circuitry

The M Series analog inputs and outputs have calibration circuitry to correct gain and offset errors. You can calibrate the device to minimize AI and AO errors caused by time and temperature drift at run time. No external circuitry is necessary; an internal reference ensures high accuracy and stability over time and temperature changes.

NI USB-621x User Manual 2-2 ni.com

Factory-calibration constants are permanently stored in an onboard EEPROM and cannot be modified. When you self-calibrate the device, software stores new constants in a user-modifiable section of the EEPROM. To return a device to its initial factory calibration settings, software can copy the factory-calibration constants to the user-modifiable section of the EEPROM. Refer to the NI-DAQmx Help or the LabVIEW 8.x Help for more information about using calibration constants.

Signal Conditioning

Many sensors and transducers require signal conditioning before a measurement system can effectively and accurately acquire the signal. The front-end signal conditioning system can include functions such as signal amplification, attenuation, filtering, electrical isolation, simultaneous sampling, and multiplexing. In addition, many transducers require excitation currents or voltages, bridge completion, linearization, or high amplification for proper and accurate operation. Therefore, most computer-based measurement systems include some form of signal conditioning in addition to plug-in data acquisition DAQ devices.

Sensors and Transducers

Sensors can generate electrical signals to measure physical phenomena, such as temperature, force, sound, or light. Some commonly used sensors are strain gauges, thermocouples, thermistors, angular encoders, linear encoders, and resistance temperature detectors (RTDs).

Chapter 2 DAQ System Overview

To measure signals from these various transducers, you must convert them into a form that a DAQ device can accept. For example, the output voltage of most thermocouples is very small and susceptible to noise. Therefore, you may need to amplify or filter the thermocouple output before digitizing it. The manipulation of signals to prepare them for digitizing is called signal conditioning.

For more information about sensors, refer to the following documents.

• For general information about sensors, visit

• If you are using LabVIEW, refer to the LabVIEW Help by selecting Help»Search the LabVIEW Help in LabVIEW and then navigate to the Taking Measurements book on the Contents tab.

• If you are using other application software, refer to Common Sensors in the NI-DAQmx Help or the LabVIEW 8.x Help.

ni.com/sensors.

Chapter 2 DAQ System Overview

Programming Devices in Software

National Instruments measurement devices are packaged with NI-DAQ driver software, an extensive library of functions and VIs you can call from your application software, such as LabVIEW or LabWindows/CVI, to program all the features of your NI measurement devices. Driver software has an application programming interface (API), which is a library of VIs, functions, classes, attributes, and properties for creating applications for your device.

NI-DAQ 7.3 and later includes two NI-DAQ drivers—Traditional NI-DAQ (Legacy) and NI-DAQmx. M Series devices use the NI-DAQmx driver. Each driver has its own API, hardware configuration, and software configuration. Refer to the NI-DAQmx for USB Devices Getting Started Guide for more information about the two drivers.

NI-DAQmx includes a collection of programming examples to help you get started developing an application. You can modify example code and save it in an application. You can use examples to develop a new application or add example code to an existing application.

To locate LabVIEW and LabWindows/CVI examples, open the National Instruments Example Finder.

• In LabVIEW, select Help»Find Examples.

• In LabWindows/CVI, select Help»NI Example Finder.

Measurement Studio, Visual Basic, and ANSI C examples are located in the following directories:

• NI-DAQmx examples for Measurement Studio-supported languages are in the following directories:

–

MeasurementStudio\VCNET\Examples\NIDaq

– MeasurementStudio\DotNET\Examples\NIDaq

• NI-DAQmx examples for ANSI C are in the

NI-DAQ\Examples\DAQmx ANSI C Dev directory

For additional examples, refer to

NI USB-621x User Manual 2-4 ni.com

zone.ni.com.

Connector Information

The I/O Connector Signal Descriptions and +5 V Power sections contain information about NI 621x connectors. Refer to Appendix A,

Device-Specific Information, for device I/O connector pinouts.

I/O Connector Signal Descriptions

Table 3-1 describes the signals found on the I/O connectors. Not all signals are available on all devices.

Table 3-1. I/O Connector Signals

Signal Name Reference Direction Description

AI GND — — Analog Input Ground—These terminals are the

reference point for single-ended AI measurements in RSE mode and the bias current return point for DIFF measurements. All three ground references—AI GND, AO GND, and D GND—are connected on the device.

AI <0..31> Va r ie s Input Analog Input Channels 0 to 31—For single-ended

measurements, each signal is an analog input voltage channel. In RSE mode, AI GND is the reference for these signals. In NRSE mode, the reference for each AI <0..31> signal is AI SENSE.

For differential measurements, AI 0 and AI 8 are the positive and negative inputs of differential analog input channel 0. Similarly, the following signal pairs also form differential input channels:

AI SENSE — Input Analog Input Sense—In NRSE mode, the reference for

each AI <0..31> signal is AI SENSE.

AO <0..1> AO GND Output Analog Output Channels 0 to 1—These terminals

supply the voltage output of AO channels 0 to 1.

Chapter 3 Connector Information

Table 3-1. I/O Connector Signals (Continued)

Signal Name Reference Direction Description

AO GND — — Analog Output Ground—AO GND is the reference for

AO <0..1>. All three ground references—AI GND, AO GND, and D GND—are connected on the device.

D GND — — Digital Ground—D GND supplies the reference for

PFI <0..15>/P0/P1 and +5 V. All three ground references—AI GND, AO GND, and D GND—are connected on the device.

+5 V D GND Input or

Output

PFI <0..3>, PFI <8..11>/P0.<0..7>

PFI <4..7>, PFI <12..15>/P1.<0..7>

NC — — No connect—Do not connect signals to these terminals.

D GND Input Programmable Function Interface or Static Digital

D GND Output Programmable Function Interface or Static Digital

+5 V Power—These terminals provide a +5 V power source or can be used to externally power the PFI outputs.

Input Channels 0 to 7—Each PFI terminal can be used to supply an external source for AI, AO, or counter/timer inputs.

You also can use these terminals as static digital input lines.

Output Channels 0 to 7—You can route many different internal AI, AO, or counter/timer outputs to each PFI terminal.

You also can use these terminals as static digital output lines.

+5 V Power

The +5 V terminals on the I/O connector can be use as either an output or an input. Both terminals are internally connected on the USB-621x.

+5 V Power as an Output

Because the USB-621x devices are bus powered, there is a 50 mA limit on the total current that can be drawn from the +5 V terminals and the digital outputs PFI <4..7> and PFI <12..15>/P1.<0..7>. The USB-621x monitors the total current and will drop the voltage on all of the digital outputs and the +5 V terminals if the 50 mA limit is exceeded.

NI USB-621x User Manual 3-2 ni.com

+5 V Power as an Input

If you have high current loads for the digital outputs to drive, you can exceed the 50 mA internal limit by connecting an external +5 V power source to the +5 V terminals. These terminals are protected against undervoltage and overvoltage, and they have a 350 mA self-resetting fuse to protect them from short circuit conditions. If your USB-621x device has more than one +5 V terminal, you can connect the external power supply to one terminal and use the other as a power source.

Chapter 3 Connector Information

Analog Input

Figure 4-1 shows the analog input circuitry of NI 621x devices.

AI <0..n>

MUX

DIFF, RSE,

AI SENSE

I/O Connector

AI GND

AI Terminal

Configuration

or NRSE

Selection

NI-PGIA

Input Range

Selection

ADC

AI FIFO

Isolation

Barrier

(USB-6215

and USB-6218

devices only)

Digital

Isolators

AI Data

Figure 4-1. M Series Analog Input Circuitry

Analog Input Circuitry

I/O Connector

You can connect analog input signals to the M Series device through the I/O connector. The proper way to connect analog input signals depends on the analog input ground-reference settings, described in the Analog Input

Ground-Reference Settings section. Also refer to Appendix A, Device-Specific Information, for device I/O connector pinouts.

MUX

Each M Series device has one analog-to-digital converter (ADC). The multiplexers (MUX) route one AI channel at a time to the ADC through the NI-PGIA.

Chapter 4 Analog Input

Ground-Reference Settings

The analog input ground-reference settings circuitry selects between differential, referenced single-ended, and non-referenced single-ended input modes. Each AI channel can use a different mode.

Instrumentation Amplifier (NI-PGIA)

The NI programmable gain instrumentation amplifier (NI-PGIA) is a measurement and instrument class amplifier that minimizes settling times for all input ranges. The NI-PGIA can amplify or attenuate an AI signal to ensure that you use the maximum resolution of the ADC.

M Series devices use the NI-PGIA to deliver high accuracy even when sampling multiple channels with small input ranges at fast rates. M Series devices can sample channels in any order at the maximum conversion rate, and you can individually program each channel in a sample with a different input range.

A/D Converter

The analog-to-digital converter (ADC) digitizes the AI signal by converting the analog voltage into a digital number.

AI FIFO

M Series devices can perform both single and multiple A/D conversions of a fixed or infinite number of samples. A large first-in-first-out (FIFO) buffer holds data during AI acquisitions to ensure that no data is lost. M Series devices can handle multiple A/D conversion operations with DMA, interrupts, or programmed I/O.

Analog Input Range

The input range affects the resolution of the M Series device for an AI channel. For example, a 16-bit ADC converts analog inputs into one of 65,536 (= 2 an input range of –10 V to 10 V, the voltage of each code of a 16-bit ADC is:

M Series devices use a calibration method that requires some codes (typically about 5% of the codes) to lie outside of the specified range. This

NI USB-621x User Manual 4-2 ni.com

) codes—that is, one of 65,536 possible digital values. So, for

(10 V – (–10 V))

= 305 μV

Chapter 4 Analog Input

calibration method improves absolute accuracy, but it increases the nominal resolution of input ranges by about 5% over what the formula shown above would indicate.

Choose an input range that matches the expected input range of your signal. A large input range can accommodate a large signal variation, but reduces the voltage resolution. Choosing a smaller input range improves the voltage resolution, but may result in the input signal going out of range.

For more information about setting ranges, refer to the NI-DAQmx Help or the LabVIEW 8.x Help.

Table 4-1 shows the input ranges and resolutions supported by NI 621x devices.

Table 4-1. Input Ranges for NI 621x

Nominal Resolution Assuming

Input Range

–10 V to 10 V 320 μV

–5 V to 5 V 160 μV

5% Over Range

–1 V to 1 V 32 μV

–200 mV to 200 mV 6.4 μV

Analog Input Ground-Reference Settings

NI 621x devices support the analog input ground-reference settings shown in Table 4-2.

Table 4-2. Analog Input Ground-Reference Settings

AI Ground-Reference

Settings

DIFF In differential (DIFF) mode, NI 621x devices measure the difference in

voltage between two AI signals.

RSE In referenced single-ended (RSE) mode, NI 621x devices measure the

voltage of an AI signal relative to AI GND.

NRSE In non-referenced single-ended (NRSE) mode, NI 621x devices measure

the voltage of an AI signal relative to the AI SENSE input.

Description

Chapter 4 Analog Input

The AI ground-reference setting determines how you should connect your AI signals to the NI 621x device. Refer to Chapter 5, Connecting AI Signals

on the USB-6210/6211 Devices, section for more information.

Ground-reference settings are programmed on a per-channel basis. For example, you might configure the device to scan 12 channels—four differentially-configured channels and eight single-ended channels.

NI 621x devices implement the different analog input ground-reference settings by routing different signals to the NI-PGIA. The NI-PGIA is a differential amplifier. That is, the NI-PGIA amplifies (or attenuates) the difference in voltage between its two inputs. The NI-PGIA drives the ADC with this amplified voltage. The amount of amplification (the gain), is determined by the analog input range, as shown in Figure 4-2.

Instrumentation

in+

Amplifier

AI Ground-Reference

Settings

RSE AI <0..31> AI GND

NRSE AI <0..31> AI SENSE

DIFF AI <0..7> AI <8..15>

] × Gain

Measured

Voltage

–

PGIA

in–

Vm = [V

in+

– V

in–

Figure 4-2. NI-PGIA

Table 4-3 shows how signals are routed to the NI-PGIA.

Table 4-3. Signals Routed to the NI-PGIA

Signals Routed to the Positive

Input of the NI-PGIA (V

in+

Signals Routed to the Negative

)

Input of the NI-PGIA (V

in–

)

AI <16..23> AI <24..31>

For differential measurements, AI 0 and AI 8 are the positive and negative inputs of differential analog input channel 0. For a complete list of signal

NI USB-621x User Manual 4-4 ni.com

pairs that form differential input channels, refer to the I/O Connector Signal

Descriptions section of Chapter 3, Connector Information.

Caution The maximum input voltages rating of AI signals with respect to AI GND (and for differential signals with respect to each other) are listed in the specifications document for your device. Exceeding the maximum input voltage of AI signals distorts the measurement results. Exceeding the maximum input voltage rating also can damage the device and the computer. NI is not liable for any damage resulting from such signal connections.

AI ground-reference setting is sometimes referred to as AI terminal configuration.

Configuring AI Ground-Reference Settings in Software

You can program channels on an M Series device to acquire with different ground references.

Chapter 4 Analog Input

To enable multimode scanning in LabVIEW, use

Virtual Channel.vi

of the NI-DAQmx API. You must use a new VI for

NI-DAQmx Create

each channel or group of channels configured in a different input mode. In Figure 4-3, channel 0 is configured in differential mode, and channel 1 is configured in RSE mode.

Figure 4-3. Enabling Multimode Scanning in LabVIEW

To configure the input mode of your voltage measurement using the DAQ Assistant, use the Terminal Configuration drop-down list. Refer to the DAQ Assistant Help for more information about the DAQ Assistant.

To configure the input mode of your voltage measurement using the NI-DAQmx C API, set the terminalConfig property. Refer to the NI-DAQmx C Reference Help for more information.

Chapter 4 Analog Input

Multichannel Scanning Considerations

M Series devices can scan multiple channels at high rates and digitize the signals accurately. However, you should consider several issues when designing your measurement system to ensure the high accuracy of your measurements.

In multichannel scanning applications, accuracy is affected by settling time. When your NI 621x device switches from one AI channel to another AI channel, the device configures the NI-PGIA with the input range of the new channel. The NI-PGIA then amplifies the input signal with the gain for the new input range. Settling time refers to the time it takes the NI-PGIA to amplify the input signal to the desired accuracy before it is sampled by the ADC. The specifications document for your DAQ device lists its settling time.

NI 621x devices are designed to have fast settling times. However, several factors can increase the settling time which decreases the accuracy of your measurements. To ensure fast settling times, you should do the following (in order of importance):

• Use low impedance sources

• Use short high-quality cabling

• Carefully choose the channel scanning order

• Avoid scanning faster than necessary

The following sections contain more information about these factors.

Use Low Impedance Sources

To ensure fast settling times, your signal sources should have an impedance of <1 kΩ. Large source impedances increase the settling time of the NI-PGIA, and so decrease the accuracy at fast scanning rates.

Settling times increase when scanning high-impedance signals due to a phenomenon called charge injection. Multiplexers contain switches, usually made of switched capacitors. When one of the channels, for example channel 0, is selected in a multiplexer, those capacitors accumulate charge. When the next channel, for example channel 1, is selected, the accumulated charge leaks backward through channel 1. If the output impedance of the source connected to channel 1 is high enough, the resulting reading of channel 1 can be partially affected by the voltage on channel 0. This effect is referred to as ghosting.

NI USB-621x User Manual 4-6 ni.com

If your source impedance is high, you can decrease the scan rate to allow the NI-PGIA more time to settle. Another option is to use a voltage follower circuit external to your DAQ device to decrease the impedance seen by the DAQ device. Refer to the KnowledgeBase document, How Do I Create a Buffer to Decrease the Source Impedance of My Analog Input Signal?, by going to

ni.com/info and entering the info code rdbbis.

Carefully Choose the Channel Scanning Order

Avoid Switching from a Large to a Small Input Range

Switching from a channel with a large input range to a channel with a small input range can greatly increase the settling time.

Suppose a 4 V signal is connected to channel 0 and a 1 mV signal is connected to channel 1. The input range for channel 0 is –10 V to 10 V and the input range of channel 1 is –200 mV to 200 mV.

When the multiplexer switches from channel 0 to channel 1, the input to the NI-PGIA switches from 4 V to 1 mV. The approximately 4 V step from 4 V to 1 mV is 1,000% of the new full-scale range. For a 16-bit device to settle within 0.0015% (15 ppm or 1 LSB) of the ±200 mV full-scale range on channel 1, the input circuitry must settle to within 0.000031% (0.31 ppm or 1/50 LSB) of the ±10 V range. Some devices can take many microseconds for the circuitry to settle this much.

Chapter 4 Analog Input

To avoid this effect, you should arrange your channel scanning order so that transitions from large to small input ranges are infrequent.

In general, you do not need this extra settling time when the NI-PGIA is switching from a small input range to a larger input range.

Insert Grounded Channel between Signal Channels

Another technique to improve settling time is to connect an input channel to ground. Then insert this channel in the scan list between two of your signal channels. The input range of the grounded channel should match the input range of the signal after the grounded channel in the scan list.

Consider again the example above where a 4 V signal is connected to channel 0 and a 1 mV signal is connected to channel 1. Suppose the input range for channel 0 is –10 V to 10 V and the input range of channel 1 is –200mV to 200mV.

Chapter 4 Analog Input

You can connect channel 2 to AI GND (or you can use the internal ground signal; refer to Internal Channels in the NI-DAQmx Help). Set the input range of channel 2 to –200 mV to 200 mV to match channel 1. Then scan channels in the order: 0, 2, 1.

Inserting a grounded channel between signal channels improves settling time because the NI-PGIA adjusts to the new input range setting faster when the input is grounded.

Minimize Voltage Step between Adjacent Channels

When scanning between channels that have the same input range, the settling time increases with the voltage step between the channels. If you know the expected input range of your signals, you can group signals with similar expected ranges together in your scan list.

For example, suppose all channels in a system use a –5 to 5 V input range. The signals on channels 0, 2, and 4 vary between 4.3 V and 5 V. The signals on channels 1, 3, and 5 vary between –4 V and 0 V. Scanning channels in the order 0, 2, 4, 1, 3, 5 produces more accurate results than scanning channels in the order 0, 1, 2, 3, 4, 5.

Avoid Scanning Faster Than Necessary

Designing your system to scan at slower speeds gives the NI-PGIA more time to settle to a more accurate level. Here are two examples to consider.

Example 1

Averaging many AI samples can increase the accuracy of the reading by decreasing noise effects. In general, the more points you average, the more accurate the final result. However, you may choose to decrease the number of points you average and slow down the scanning rate.

Suppose you want to sample 10 channels over a period of 20 ms and average the results. You could acquire 250 points from each channel at a scan rate of 125 kS/s. Another method would be to acquire 500 points from each channel at a scan rate of 250 kS/s. Both methods take the same amount of time. Doubling the number of samples averaged (from 250 to 500) decreases the effect of noise by a factor of 1.4 (the square root of 2). However, doubling the number of samples (in this example) decreases the time the NI-PGIA has to settle from 8 µs to 4 µs. In some cases, the slower scan rate system returns more accurate results.

NI USB-621x User Manual 4-8 ni.com

Example 2

If the time relationship between channels is not critical, you can sample from the same channel multiple times and scan less frequently. For example, suppose an application requires averaging 100 points from channel 0 and averaging 100 points from channel 1. You could alternate reading between channels—that is, read one point from channel 0, then one point from channel 1, and so on. You also could read all 100 points from channel 0 then read 100 points from channel 1. The second method switches between channels much less often and is affected much less by settling time.

Analog Input Data Acquisition Methods

When performing analog input measurements, you either can perform software-timed or hardware-timed acquisitions. Hardware-timed acquisitions can be buffered or non-buffered.

Software-Timed Acquisitions

With a software-timed acquisition, software controls the rate of the acquisition. Software sends a separate command to the hardware to initiate each ADC conversion. In NI-DAQmx, software-timed acquisitions are referred to as having on-demand timing. Software-timed acquisitions are also referred to as immediate or static acquisitions and are typically used for reading a single sample of data.

Chapter 4 Analog Input

Hardware-Timed Acquisitions

With hardware-timed acquisitions, a digital hardware signal (ai/SampleClock) controls the rate of the acquisition. This signal can be generated internally on your device or provided externally.

Hardware-timed acquisitions have several advantages over software-timed acquisitions.

• The time between samples can be much shorter.

• The timing between samples is deterministic.

• Hardware-timed acquisitions can use hardware triggering.

Hardware-timed operations can be buffered or non-buffered.

Chapter 4 Analog Input

Buffered

In a buffered acquisition, data is moved from the DAQ device’s onboard FI FO memory to a PC bu ffer u s ing USB signal streams or programmed I/O before it is transferred to application memory. Buffered acquisitions typically allow for much faster transfer rates than non-buffered acquisitions because data is moved in large blocks, rather than one point at a time.

One property of buffered I/O operations is the sample mode. The sample mode can be either finite or continuous.

Finite sample mode acquisition refers to the acquisition of a specific, predetermined number of data samples. Once the specified number of samples has been written out, the generation stops. If you use a reference trigger, you must use finite sample mode.

Continuous acquisition refers to the acquisition of an unspecified number of samples. Instead of acquiring a set number of data samples and stopping, a continuous acquisition continues until you stop the operation. Continuous acquisition is also referred to as double-buffered or circular-buffered acquisition.

If data cannot be transferred across the bus fast enough, the FIFO becomes full. New acquisitions will overwrite data in the FIFO before it can be transferred to host memory. The device generates an error in this case. With continuous operations, if the user program does not read data out of the PC buffer fast enough to keep up with the data transfer, the buffer could reach an overflow condition, causing an error to be generated.

Non-Buffered

Hardware-timed, non-buffered mode is not supported for USB M series devices.

Analog Input Digital Triggering

Analog input supports three different triggering actions:

• Start trigger

• Reference trigger

• Pause trigger

Refer to the AI Start Trigger Signal, AI Reference Trigger Signal, and AI

Pause Trigger Signal sections for information about these triggers.

NI USB-621x User Manual 4-10 ni.com

A digital trigger can initiate these actions. All NI 621x devices support digital triggering. NI 621x devices do not support analog triggering.

Field Wiring Considerations

Environmental noise can seriously affect the measurement accuracy of the device if you do not take proper care when running signal wires between signal sources and the device. The following recommendations apply mainly to AI signal routing to the device, although they also apply to signal routing in general.

Minimize noise pickup and maximize measurement accuracy by taking the following precautions.

• Use DIFF AI connections to reject common-mode noise.

• Use individually shielded, twisted-pair wires to connect AI signals to the device. With this type of wire, the signals attached to the positive and negative input channels are twisted together and then covered with a shield. You then connect this shield only at one point to the signal source ground. This kind of connection is required for signals traveling through areas with large magnetic fields or high electromagnetic interference.

Chapter 4 Analog Input

Refer to the NI Developer Zone document, Field Wiring and Noise Considerations for Analog Signals, for more information. To access this

document, go to

ni.com/info and enter the info code rdfwn3.

Analog Input Timing Signals

In order to provide all of the timing functionality described throughout this section, M Series devices have a flexible timing engine. Figure 4-4 summarizes all of the timing options provided by the analog input timing engine.

Chapter 4 Analog Input

20 MHz Timebase

100 kHz Timebase

ai/SampleClock

Timebase

Analog Comparison Event

Ctr

Internal Output

SW Pulse

Programmable

Clock

Divider

ai/SampleClock

ai/ConvertClock

Timebase

Internal Output

Ctr

Programmable

Clock

Divider

Figure 4-4. Analog Input Timing Options

M Series devices use ai/SampleClock and ai/ConvertClock to perform interval sampling. As Figure 4-5 shows, ai/SampleClock controls the sample period, which is determined by the following equation:

1/Sample Period = Sample Rate

Channel 0

Channel 1

Convert Period

Sample Period

Figure 4-5. Interval Sampling

ai/ConvertClock controls the Convert Period, which is determined by the following equation:

1/Convert Period = Convert Rate

NI USB-621x User Manual 4-12 ni.com

Chapter 4 Analog Input

NI-DAQmx chooses the default convert rate to allow for the maximum settling time between conversions. Typically, this rate is the sampling rate for the task multiplied by the number of channels in the task.

Note The sampling rate is the fastest you can acquire data on the device and still achieve accurate results. For example, if an M Series device has a sampling rate of 250 kS/s, this sampling rate is aggregate—one channel at 250 kS/s or two channels at 125 kS/s per channel illustrates the relationship.

Posttriggered data acquisition allows you to view only data that is acquired after a trigger event is received. A typical posttriggered DAQ sequence is shown in Figure 4-6. The sample counter is loaded with the specified number of posttrigger samples, in this example, five. The value decrements with each pulse on ai/SampleClock, until the value reaches zero and all desired samples have been acquired.

ai/StartTrigger

ai/SampleClock

ai/ConvertClock

Sample Counter

Figure 4-6. Posttriggered Data Acquisition Example

13042

Pretriggered data acquisition allows you to view data that is acquired before the trigger of interest, in addition to data acquired after the trigger. Figure 4-7 shows a typical pretriggered DAQ sequence. ai/StartTrigger can be either a hardware or software signal. If ai/StartTrigger is set up to be a software start trigger, an output pulse appears on the ai/StartTrigger line when the acquisition begins. When the ai/StartTrigger pulse occurs, the sample counter is loaded with the number of pretriggered samples, in this example, four. The value decrements with each pulse on ai/SampleClock, until the value reaches zero. The sample counter is then loaded with the number of posttriggered samples, in this example, three.

Chapter 4 Analog Input

ai/StartTrigger

ai/ReferenceTrigger

ai/SampleClock

ai/ConvertClock

Scan Counter

Figure 4-7. Pretriggered Data Acquisition Example

n/a

0 1 2

1 0 2 2 2

If an ai/ReferenceTrigger pulse occurs before the specified number of pretrigger samples are acquired, the trigger pulse is ignored. Otherwise, when the ai/ReferenceTrigger pulse occurs, the sample counter value decrements until the specified number of posttrigger samples have been acquired.

USB M Series devices feature the following analog input timing signals.

• AI Sample Clock Signal

• AI Sample Clock Timebase Signal

• AI Convert Clock Signal

• AI Convert Clock Timebase Signal

• AI Hold Complete Event Signal

• AI Start Trigger Signal

• AI Reference Trigger Signal

• AI Pause Trigger Signal

AI Sample Clock Signal

Use the AI Sample Clock (ai/SampleClock) signal to initiate a set of measurements. Your M Series device samples the AI signals of every channel in the task once for every ai/SampleClock. A measurement acquisition consists of one or more samples.

You can specify an internal or external source for ai/SampleClock. You also can specify whether the measurement sample begins on the rising edge or falling edge of ai/SampleClock.

NI USB-621x User Manual 4-14 ni.com

Chapter 4 Analog Input

Using an Internal Source

One of the following internal signals can drive ai/SampleClock.

• Counter n Internal Output

• AI Sample Clock Timebase (divided down)

•A software pulse

A programmable internal counter divides down the sample clock timebase.

Using an External Source

Use the external signals PFI <0..3> or PFI <8..11> as the source of ai/SampleClock.

Routing AI Sample Clock Signal to an Output Terminal

You can route ai/SampleClock out to any PFI <4..7> or PFI <12..15> terminal. This pulse is always active high.

You can specify the output to have one of two behaviors. With the pulse behavior, your DAQ device briefly pulses the PFI terminal once for every occurrence of ai/SampleClock.

With level behavior, your DAQ device drives the PFI terminal high during the entire sample.

Other Timing Requirements

Your DAQ device only acquires data during an acquisition. The device ignores ai/SampleClock when a measurement acquisition is not in progress. During a measurement acquisition, you can cause your DAQ device to ignore ai/SampleClock using the ai/PauseTrigger signal.

A counter on your device internally generates ai/SampleClock unless you select some external source. ai/StartTrigger starts this counter and either software or hardware can stop it once a finite acquisition completes. When using an internally generated ai/SampleClock, you also can specify a configurable delay from ai/StartTrigger to the first ai/SampleClock pulse. By default, this delay is set to two ticks of the ai/SampleClockTimebase signal. When using an externally generated ai/SampleClock, you must ensure the clock signal is consistent with respect to the timing requirements of ai/ConvertClock. Failure to do so may result in ai/SampleClock pulses that are masked off and acquisitions with erratic sampling intervals. Refer

Chapter 4 Analog Input

to AI Convert Clock Signal for more information about the timing requirements between ai/ConvertClock and ai/SampleClock.

Figure 4-8 shows the relationship of ai/SampleClock to ai/StartTrigger.

ai/SampleClockTimebase

Figure 4-8. ai/SampleClock and ai/StartTrigger

AI Sample Clock Timebase Signal

You can route any of the following signals to be the AI Sample Clock Timebase (ai/SampleClockTimebase) signal:

• 20 MHz Timebase

• 100 kHz Timebase

• PFI <0..3>, PFI <8..11>

ai/StartTrigger

ai/SampleClock

Delay

From

Start

Trigger

ai/SampleClockTimebase is not available as an output on the I/O connector. ai/SampleClockTimebase is divided down to provide one of the possible sources for ai/SampleClock. You can configure the polarity selection for ai/SampleClockTimebase as either rising or falling edge.

AI Convert Clock Signal

Use the AI Convert Clock (ai/ConvertClock) signal to initiate a single A/D conversion on a single channel. A sample (controlled by the AI Sample Clock) consists of one or more conversions.

You can specify either an internal or external signal as the source of ai/ConvertClock. You also can specify whether the measurement sample begins on the rising edge or falling edge of ai/ConvertClock.

By default, NI-DAQmx chooses the fastest conversion rate possible based on the speed of the A/D converter and adds 10 µs of padding between each

NI USB-621x User Manual 4-16 ni.com

Chapter 4 Analog Input

channel to allow for adequate settling time. This scheme enables the channels to approximate simultaneous sampling and still allow for adequate settling time. If the AI Sample Clock rate is too fast to allow for this 10 µs of padding, NI-DAQmx chooses the conversion rate so that the AI Convert Clock pulses are evenly spaced throughout the sample.

To explicitly specify the conversion rate, use AI Convert Clock Rate DAQmx Timing property node or function.

Caution Setting the conversion rate higher than the maximum rate specified for your device will result in errors.

Using an Internal Source

One of the following internal signals can drive ai/ConvertClock:

• AI Convert Clock Timebase (divided down)

• Counter n Internal Output

A programmable internal counter divides down the AI Convert Clock Timebase to generate ai/ConvertClock. The counter is started by ai/SampleClock and continues to count down to zero, produces an ai/ConvertClock, reloads itself, and repeats the process until the sample is finished. It then reloads itself in preparation for the next ai/SampleClock pulse.

Using an External Source

Use the external signals PFI <0..3> or PFI <8..11> as the source of ai/ConvertClock:

Routing AI Convert Clock Signal to an Output Terminal

You can route ai/ConvertClock (as an active low signal) out to any PFI <4..7> or PFI <12..15> terminal.

Using a Delay from Sample Clock to Convert Clock

When using an internally generated ai/ConvertClock, you also can specify a configurable delay from ai/SampleClock to the first ai/ConvertClock pulse within the sample. By default, this delay is three ticks of ai/ConvertClockTimebase.

Chapter 4 Analog Input

ai/ConvertClockTimebase

ai/SampleClock

ai/ConvertClock

Figure 4-9 shows the relationship of ai/SampleClock to ai/ConvertClock.

Delay

From

Sample

Clock

Convert

Perio d

Figure 4-9. ai/SampleClock and ai/ConvertClock

Other Timing Requirements

The sample and conversion level timing of M Series devices work such that clock signals are gated off unless the proper timing requirements are met. For example, the device ignores both ai/SampleClock and ai/ConvertClock until it receives a valid ai/StartTrigger signal. Once the device recognizes an ai/SampleClock pulse, it ignores subsequent ai/SampleClock pulses until it receives the correct number of ai/ConvertClock pulses.

Similarly, the device ignores all ai/ConvertClock pulses until it recognizes an ai/SampleClock pulse. Once the device receives the correct number of ai/ConvertClock pulses, it ignores subsequent ai/ConvertClock pulses until it receives another ai/SampleClock. Figures 4-10, 4-11, 4-12, and 4-13 show timing sequences for a four-channel acquisition (using AI channels 0, 1, 2, and 3) and demonstrate proper and improper sequencing of ai/SampleClock and ai/ConvertClock.

NI USB-621x User Manual 4-18 ni.com

ai/SampleClock

ai/ConvertClock

Channel Measured 1 2 3 0

• Sample Clock Too Fast for Convert Clock

• Sample Clock Pulses are Gated Off

Figure 4-10. ai/SampleClock Too Fast

ai/SampleClock

ai/ConvertClock

Chapter 4 Analog Input

1 2 3 0 1 2 3 0

Sample #1 Sample #2 Sample #3

ai/SampleClock

ai/ConvertClock

Channel Measured

• Convert Clock Too Fast for Sample Clock

• Convert Clock Pulses are Gated Off

1230

Sample #1 Sample #2 Sample #3

Figure 4-11. ai/ConvertClock Too Fast

1230

Sample #1 Sample #2 Sample #3

• Improperly Matched Sample Clock and Convert Clock

• Leads to Aperiodic Sampling

1230

Figure 4-12. ai/SampleClock and ai/ConvertClock Improperly Matched

12301230

Chapter 4 Analog Input

ai/SampleClock

ai/ConvertClock

Channel Measured 1 2 3 0

Sample #1 Sample #2 Sample #3

• Properly Matched Sample Clock and Convert Clock

Figure 4-13. ai/SampleClock and ai/ConvertClock Properly Matched

1 2 3 0 1 2 3 0

It is also possible to use a single external signal to drive both ai/SampleClock and ai/ConvertClock at the same time. In this mode, each tick of the external clock will cause a conversion on the ADC. Figure 4-14 shows this timing relationship.

ai/SampleClock

ai/ConvertClock

Channel Measured

• One External Signal Driving Both Clocks

Figure 4-14. Single External Signal Driving ai/SampleClock and ai/ConvertClock

1230

Sample #1 Sample #2 Sample #3

Simultaneously

12301…0

AI Convert Clock Timebase Signal

The AI Convert Clock Timebase (ai/ConvertClockTimebase) signal is divided down to provide on of the possible sources for ai/ConvertClock. Use one of the following signals as the source of ai/ConvertClockTimebase:

• ai/SampleClockTimebase

• 20 MHz Timebase

ai/ConvertClockTimebase is not available as an output on the I/O connector.

NI USB-621x User Manual 4-20 ni.com

AI Hold Complete Event Signal

The AI Hold Complete Event (ai/HoldCompleteEvent) signal generates a pulse after each A/D conversion begins. You can route ai/HoldCompleteEvent out to any PFI <4..8> or PFI <12..15> terminal.

The polarity of ai/HoldCompleteEvent is software-selectable, but is typically configured so that a low-to-high leading edge can clock external AI multiplexers indicating when the input signal has been sampled and can be removed.

AI Start Trigger Signal

Use the AI Start Trigger (ai/StartTrigger) signal to begin a measurement acquisition. A measurement acquisition consists of one or more samples. If you do not use triggers, begin a measurement with a software command. Once the acquisition begins, configure the acquisition to stop:

• When a certain number of points are sampled (in finite mode)

• After a hardware reference trigger (in finite mode)

• With a software command (in continuous mode)

An acquisition that uses a start trigger (but not a reference trigger) is sometimes referred to as a posttriggered acquisition.

Chapter 4 Analog Input

Using a Digital Source

To use ai/StartTrigger with a digital source, specify a source and an edge. The source can be any of the following signals:

• PFI <0..3>, PFI <8..11>

• Counter n Internal Output

The source also can be one of several other internal signals on your DAQ device. Refer to Device Routing in MAX in the NI-DAQmx Help or the LabVIEW 8.x Help for more information.

You also can specify whether the measurement acquisition begins on the rising edge or falling edge of ai/StartTrigger.

Routing AI Start Trigger to an Output Terminal

You can route ai/StartTrigger out to any PFI <4..8> or PFII <12..15> terminal.

The output is an active high pulse.

Chapter 4 Analog Input

The device also uses ai/StartTrigger to initiate pretriggered DAQ operations. In most pretriggered applications, a software trigger generates ai/StartTrigger. Refer to the AI Reference Trigger Signal section for a complete description of the use of ai/StartTrigger and ai/ReferenceTrigger in a pretriggered DAQ operation.

AI Reference Trigger Signal

Use a reference trigger (ai/ReferenceTrigger) signal to stop a measurement acquisition. To use a reference trigger, specify a buffer of finite size and a number of pretrigger samples (samples that occur before the reference trigger). The number of posttrigger samples (samples that occur after the reference trigger) desired is the buffer size minus the number of pretrigger samples.

Once the acquisition begins, the DAQ device writes samples to the buffer. After the DAQ device captures the specified number of pretrigger samples, the DAQ device begins to look for the reference trigger condition. If the reference trigger condition occurs before the DAQ device captures the specified number of pretrigger samples, the DAQ device ignores the condition.

If the buffer becomes full, the DAQ device continuously discards the oldest samples in the buffer to make space for the next sample. This data can be accessed (with some limitations) before the DAQ device discards it. Refer to the KnowledgeBase document, Can a Pretriggered Acquisition be Continuous?, for more information. To access this KnowledgeBase, go to

ni.com/info and enter the info code rdcanq.

When the reference trigger occurs, the DAQ device continues to write samples to the buffer until the buffer contains the number of posttrigger samples desired. Figure 4-15 shows the final buffer.

Reference Trigger

Pretrigger Samples

Complete Buffer

Figure 4-15. Reference Trigger Final Buffer

Posttrigger Samples

NI USB-621x User Manual 4-22 ni.com

Using a Digital Source

To use ai/ReferenceTrigger with a digital source, specify a source and an edge. The source can be the PFI <0..3> or PFI <8..11> signals.

The source also can be one of several internal signals on your DAQ device. Refer to Device Routing in MAX in the NI-DAQmx Help or the LabVIEW 8.x Help for more information.

You also can specify whether the measurement acquisition stops on the rising edge or falling edge of ai/ReferenceTrigger.

Routing AI Reference Trigger Signal to an Output Terminal

You can route ai/ReferenceTrigger out to any PFI <4..7> or PFI <12..15> terminal.

AI Pause Trigger Signal

You can use the AI Pause Trigger (ai/PauseTrigger) signal to pause and resume a measurement acquisition. The internal sample clock pauses while the external trigger signal is active and resumes when the signal is inactive. You can program the active level of the pause trigger to be high or low.

Chapter 4 Analog Input

Using a Digital Source

To use ai/SampleClock, specify a source and a polarity. The source can be the PFI <0..3> or PFI <8..11> signals.

The source also can be one of several other internal signals on your DAQ device. Refer to Device Routing in MAX in the NI-DAQmx Help or the LabVIEW 8.x Help for more information.

Chapter 4 Analog Input

Getting Started with AI Applications in Software

You can use the M Series device in the following analog input applications.

• Single-point analog input

• Finite analog input

• Continuous analog input

You can perform these applications through DMA, interrupt, or programmed I/O data transfer mechanisms. Some of the applications also use start, reference, and pause triggers.

Note For more information about programming analog input applications and triggers in software, refer to the NI-DAQmx Help or the LabVIEW 8.x Help.

NI USB-621x User Manual 4-24 ni.com

Connecting AI Signals on the USB-6210/6211 Devices

Table 5-1 summarizes the recommended input configuration for both types of signal sources on NI 6210/6211 devices.

Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

Table 5-1. Analog Input Configuration

Ground-Reference

Setting

Differential

Single-Ended— Non-Referenced (NRSE)

Floating Signal Sources (Not

Connected to Building Ground)

Examples:

• Ungrounded thermocouples

• Signal conditioning with isolated outputs

• Battery devices

Signal Source DAQ Device

AI+

–

Signal Source DAQ Device

+ –

AI–

–

AI GND

–

AI SENSE

AI GND

Ground-Referenced Signal

Sources

Example:

• Plug-in instruments with non-isolated outputs

Signal Source DAQ Device

AI+

+ –

Signal Source DAQ Device

+ –

AI–

–

AI GND

–

AI SENSE

AI GND

Single-Ended— Referenced (RSE)

Refer to the Analog Input Ground-Reference Settings section of Chapter 4, Analog Input, for descriptions of the RSE,

Signal Source DAQ Device

–

AI GND

NOT RECOMMENDED

for the

USB-6210/6211

Signal Source DAQ Device

+ –

Ground-loop potential (VA – VB) are added

to measured signal.

–

AI GND

NRSE, and DIFF modes and software considerations.

Refer to the Connecting Ground-Referenced Signal Sources section for more information.

NI USB-621x User Manual 5-2 ni.com

Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

Connecting Floating Signal Sources

What Are Floating Signal Sources?

A floating signal source is not connected to the building ground system, but has an isolated ground-reference point. Some examples of floating signal sources are outputs of transformers, thermocouples, battery-powered devices, optical isolators, and isolation amplifiers. An instrument or device that has an isolated output is a floating signal source.

When to Use Differential Connections with Floating Signal Sources

Use DIFF input connections for any channel that meets any of the following conditions:

• The input signal is low level (less than 1 V).

• The leads connecting the signal to the device are greater than 3m(10ft).

• The input signal requires a separate ground-reference point or return signal.

• The signal leads travel through noisy environments.

• Two analog input channels, AI+ and AI–, are available for the signal.

DIFF signal connections reduce noise pickup and increase common-mode noise rejection. DIFF signal connections also allow input signals to float within the common-mode limits of the NI-PGIA.

Refer to the Using Differential Connections for Floating Signal Sources section for more information about differential connections.

When to Use Referenced Single-Ended (RSE) Connections with Floating Signal Sources

Only use RSE input connections if the input signal meets the following conditions.

• The input signal can share a common reference point, AI GND, with other signals that use RSE.

• The input signal is high-level (greater than 1 V).

• The leads connecting the signal to the device are less than 3 m (10 ft).

DIFF input connections are recommended for greater signal integrity for any input signal that does not meet the preceding conditions.

Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

In the single-ended modes, more electrostatic and magnetic noise couples into the signal connections than in DIFF configurations. The coupling is the result of differences in the signal path. Magnetic coupling is proportional to the area between the two signal conductors. Electrical coupling is a function of how much the electric field differs between the two conductors.

With this type of connection, the NI-PGIA rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the device ground.

Refer to the Using Referenced Single-Ended (RSE) Connections for

Floating Signal Sources section for more information about RSE

connections.

When to Use Non-Referenced Single-Ended (NRSE) Connections with Floating Signal Sources

Only use NRSE input connections if the input signal meets the following conditions.

• The input signal is high-level (greater than 1 V).

• The leads connecting the signal to the device are less than 3 m (10 ft).

DIFF input connections are recommended for greater signal integrity for any input signal that does not meet the preceding conditions.

With this type of connection, the NI-PGIA rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the device ground.

Refer to the Using Non-Referenced Single-Ended (NRSE) Connections for

Floating Signal Sources section for more information about NRSE

connections.

NI USB-621x User Manual 5-4 ni.com

Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

Using Differential Connections for Floating Signal Sources

It is important to connect the negative lead of a floating source to AI GND (either directly or through a bias resistor). Otherwise, the source may float out of the maximum working voltage range of the NI-PGIA and the DAQ device returns erroneous data.

The easiest way to reference the source to AI GND is to connect the positive side of the signal to AI+ and connect the negative side of the signal to AI GND as well as to AI– without using resistors. This connection works well for DC-coupled sources with low source impedance (less than 100 Ω).

AI+

Floating

Signal

Source

Inpedance <100 Ω

Figure 5-1. Differential Connections for Floating Signal Sources without Bias Resistors

–

AI–

AI SENSE

AI GND

However, for larger source impedances, this connection leaves the DIFF signal path significantly off balance. Noise that couples electrostatically onto the positive line does not couple onto the negative line because it is connected to ground. This noise appears as a DIFF-mode signal instead of a common-mode signal, and thus appears in your data. In this case, instead of directly connecting the negative line to AI GND, connect the negative line to AI GND through a resistor that is about 100 times the equivalent source impedance. The resistor puts the signal path nearly in balance, so that about the same amount of noise couples onto both connections, yielding better rejection of electrostatically coupled noise. This configuration does not load down the source (other than the very high input impedance of the NI-PGIA).

Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

Floating

Signal

Source

R is about 100 times source impedance of sensor

–

AI+

AI–

AI SENSE

AI GND

Figure 5-2. Differential Connections for Floating Signal Sources with Single Bias

Resistor

You can fully balance the signal path by connecting another resistor of the same value between the positive input and AI GND on the USB-621x device, as shown in Figure 5-3. This fully balanced configuration offers slightly better noise rejection, but has the disadvantage of loading the source down with the series combination (sum) of the two resistors. If, for example, the source impedance is 2 kΩ and each of the two resistors is 100 kΩ, the resistors load down the source with 200 kΩ and produce a –1% gain error.

NI USB-621x User Manual 5-6 ni.com

Floating

Signal

Source

Bias

Current

Return

Path s

–

Bias Resistors (see text)

Input Multiplexers

AI SENSE

Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

AI+

Instrumentation

Amplifier

PGIA

AI–

–

Measured

Voltage

I/O Connector

AI GND

M Series Device Configured in DIFF Mode

Figure 5-3. Differential Connections for Floating Signal Sources with Balanced Bias

Resistors

Both inputs of the NI-PGIA require a DC path to ground in order for the NI-PGIA to work. If the source is AC coupled (capacitively coupled), the NI-PGIA needs a resistor between the positive input and AI GND. If the source has low-impedance, choose a resistor that is large enough not to significantly load the source but small enough not to produce significant input offset voltage as a result of input bias current (typically 100 kΩ to 1MΩ). In this case, connect the negative input directly to AI GND. If the source has high output impedance, balance the signal path as previously described using the same value resistor on both the positive and negative

Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

inputs; be aware that there is some gain error from loading down the source, as shown in Figure 5-4.

AC Coupled

Floating

Signal

Source

AC Coupling

–

AI+

AI–

AI SENSE

AI GND

Figure 5-4. Differential Connections for AC Coupled Floating Sources

with Balanced Bias Resistors

Using Non-Referenced Single-Ended (NRSE) Connections for Floating Signal Sources

It is important to connect the negative lead of a floating signals source to AI GND (either directly or through a resistor). Otherwise the source may float out of the valid input range of the NI-PGIA and the DAQ device returns erroneous data.

Figure 5-5 shows a floating source connected to the DAQ device in NRSE mode.

Floating

Signal

Source

–

AI SENSE

AI GND

Figure 5-5. NRSE Connections for Floating Signal Sources

All of the bias resistor configurations discussed in the Using Differential

Connections for Floating Signal Sources section apply to the NRSE bias

resistors as well. Replace AI– with AI SENSE in Figures 5-1, 5-2, 5-3, and 5-4 for configurations with zero to two bias resistors. The noise rejection of NRSE mode is better than RSE mode because the AI SENSE connection is made remotely near the source. However, the noise rejection of NRSE mode is worse than DIFF mode because the AI SENSE

NI USB-621x User Manual 5-8 ni.com

Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

connection is shared with all channels rather than being cabled in a twisted pair with the AI+ signal.

Using the DAQ Assistant, you can configure the channels for RSE or NRSE input modes. Refer to the Configuring AI Ground-Reference Settings in

Software section of Chapter 4, Analog Input, for more information about

the DAQ Assistant.

Using Referenced Single-Ended (RSE) Connections for Floating Signal Sources

Figure 5-6 shows how to connect a floating signal source to the NI 621x device configured for RSE mode.

AI <0..

Programmable Gain

Floating

Signal

Source

–

I/O Connector

Input Multiplexers

AI SENSE

AI GND

Selected Channel in RSE Configuration

Instrumentation

PGIA

–

Amplifier

Measured

Voltage

–

Figure 5-6. RSE Connections for Floating Signal Sources

Using the DAQ Assistant, you can configure the channels for RSE or NRSE input modes. Refer to the Configuring AI Ground-Reference Settings in

Software section of Chapter 4, Analog Input, for more information about

the DAQ Assistant.

Connecting Ground-Referenced Signal Sources

What Are Ground-Referenced Signal Sources?

A ground-referenced signal source is a signal source connected to the building system ground. It is already connected to a common ground point with respect to the device, assuming that the computer is plugged into the same power system as the source. Non-isolated outputs of instruments and devices that plug into the building power system fall into this category.

Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

The difference in ground potential between two instruments connected to the same building power system is typically between 1 and 100 mV, but the difference can be much higher if power distribution circuits are improperly connected. If a grounded signal source is incorrectly measured, this difference can appear as measurement error. Follow the connection instructions for grounded signal sources to eliminate this ground potential difference from the measured signal.

When to Use Differential Connections with Ground-Referenced Signal Sources

Use DIFF input connections for any channel that meets any of the following conditions:

• The input signal is low level (less than 1 V).

• The leads connecting the signal to the device are greater than 3m(10ft).

• The input signal requires a separate ground-reference point or return signal.

• The signal leads travel through noisy environments.

• Two analog input channels, AI+ and AI–, are available.

DIFF signal connections reduce noise pickup and increase common-mode noise rejection. DIFF signal connections also allow input signals to float within the common-mode limits of the NI-PGIA.

Refer to the Using Differential Connections for Ground-Referenced Signal

Sources section for more information about differential connections.

When to Use Non-Referenced Single-Ended (NRSE) Connections with Ground-Referenced Signal Sources

Only use non-referenced single-ended input connections if the input signal meets the following conditions.

• The input signal is high-level (greater than 1 V).

• The leads connecting the signal to the device are less than 3 m (10 ft).

• The input signal can share a common reference point with other signals.

DIFF input connections are recommended for greater signal integrity for any input signal that does not meet the preceding conditions.

NI USB-621x User Manual 5-10 ni.com

Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

With this type of connection, the NI-PGIA rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the device ground.

Refer to the Using Non-Referenced Single-Ended (NRSE) Connections for

Ground-Referenced Signal Sources section for more information about

NRSE connections.

When to Use Referenced Single-Ended (RSE) Connections with Ground-Referenced Signal Sources

Do not use RSE connections with ground-referenced signal sources. Use NRSE or DIFF connections instead.

As shown in the bottom-rightmost cell of Table 5-1, there can be a potential difference between AI GND and the ground of the sensor. In RSE mode, this ground loop causes measurement errors.

Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

Using Differential Connections for Ground-Referenced Signal Sources

Figure 5-7 shows how to connect a ground-referenced signal source to the USB-6210/6211 device configured in DIFF mode.

Ground-

Referenced

Signal

Source

Common-

Mode

Noise and

Ground

Potential

I/O Connector

–

AI–

–

Input Multiplexers

AI SENSE

AI GND

M Series Device Configured in DIFF Mode

Instrumentation

Amplifier

PGIA

–

Measured

Voltage

–

Figure 5-7. Differential Connections for Ground-Referenced Signal Sources

With this type of connection, the NI-PGIA rejects both the common-mode noise in the signal and the ground potential difference between the signal source and the device ground, shown as V

in the figure.

AI+ and AI– must both remain within ±11 V of AI GND.

NI USB-621x User Manual 5-12 ni.com

Chapter 5 Connecting AI Signals on the USB-6210/6211 Devices

Using Non-Referenced Single-Ended (NRSE) Connections for Ground-Referenced Signal Sources

Figure 5-8 shows how to connect ground-reference signal sources to the USB-6210/6211 device in NRSE mode.

I/O Connector

Ground-

Referenced

Signal

Source

Common-

Mode Noise

and Ground

Potential

AI <0..15>

or AI <16..

–

Input Multiplexers

AI GND

–

M Series Device Configured in NRSE Mode

AI SENSE

Instrumentation

Amplifier

PGIA

–

Measured

Figure 5-8. Single-Ended Connections for Ground-Referenced Signal Sources

(NRSE Configuration)

AI+ and AI– must both remain within ±11 V of AI GND.

To measure a single-ended, ground-referenced signal source, you must use the NRSE ground-reference setting. Connect the signal to one of AI <0..31> and connect the signal local ground reference to AI SENSE. AI SENSE is internally connected to the negative input of the NI-PGIA. Therefore, the ground point of the signal connects to the negative input of the NI-PGIA.

Any potential difference between the device ground and the signal ground appears as a common-mode signal at both the positive and negative inputs of the NI-PGIA, and this difference is rejected by the amplifier. If the input circuitry of a device were referenced to ground, as it is in the RSE ground-reference setting, this difference in ground potentials would appear as an error in the measured voltage.

Using the DAQ Assistant, you can configure the channels for RSE or NRSE input modes. Refer to the Configuring AI Ground-Reference Settings in

Software section of Chapter 4, Analog Input, for more information about

the DAQ Assistant.

Voltage

–

Connecting AI Signals on the USB-6215/6218 Devices

You can connect the USB-6215/6218 directly to a variety of devices and other signal sources. Make sure the devices you connect to the USB-6215/6218 are compatible with the input specifications of the module.

When connecting various sources to the USB-6215/6218, you can use differential, single-ended, or a combination of single-ended and differential connections. Refer to Figures 6-1, 6-2, and 6-3 for diagrams of each connection type.

Note You must always connect AI GND to a local ground signal in your system using a low impedance connection. If you leave AI GND unconnected, you cannot ensure that AI <0..31> are within 10 V of AI GND, and your measurement may be unreliable.

Differential Measurements

To attain more accurate measurements and less noise, use a differential measurement configuration. A differential measurement configuration requires two inputs for each measurement.

Differential Pairs

Table 6-1 the signal pairs that are valid for differential connection configurations with the USB-6215/6218.

Table 6-1. I/O Connector Signals

Channel Signal + Signal –

0 AI 0 AI 8

1 AI 1 AI 9

2 AI 2 AI 10

Chapter 6 Connecting AI Signals on the USB-6215/6218 Devices

Table 6-1. I/O Connector Signals (Continued)

Channel Signal + Signal –

3 A I3 AI 11

4 A I4 AI 12

5 A I5 AI 13

6 A I6 AI 14

7 A I7 AI 15

16* AI 16 AI 24

17* AI 17 AI 25

18* AI 18 AI 26

19* AI 19 AI 27

20* AI 20 AI 28

21* AI 21 AI 29

22* AI 22 AI 30

23* AI 23 AI 31

* USB-6218 devices only.

Refer to Figure 6-1 for an illustration of a differential connection configuration.

AI0+

AI0– (AI8)

AI1+

AI1– (AI9)

AI GND

This signal name indicates the differential pair. Refer to Table1 for a list of differential signal pairs.

Figure 6-1. Connecting a Device to a USB-6215/6218 Using Differential Connections

NI USB-621x User Manual 6-2 ni.com

PGIA

MUX

ADC

USB-6215/6218

Chapter 6 Connecting AI Signals on the USB-6215/6218 Devices

The differential connection configuration allows the common-mode noise voltage, V

, to be rejected during the measurement of V1.

You must connect the negative lead of your sensors and AI GND to a local ground signal on your system.

Referenced Single-Ended (RSE) Measurements

Using the RSE measurement configuration allows the USB-6215/6218 to take measurements on all AI channels when all channels share a common ground. Refer to Figure 6-2 for an illustration of an RSE connection configuration.

Note If you leave the AI GND pin unconnected, the signals will float outside the working input range of the USB-6215/6218, which may result in unreliable measurements because there is no way to ensure that the input signal is within 10 V of AI GND.

AI1

AI2

MUX

PGIA

ADC

AI GND

USB-6215/6218

Figure 6-2. Connecting a Device to a USB-6215/6218 Using RSE Connections

In an RSE connection configuration, each input channel is measured with respect to AI GND.

Chapter 6 Connecting AI Signals on the USB-6215/6218 Devices

Non-Referenced Single-Ended (NRSE) Measurements

To reach a compromise between RSE and differential measurements, you can use an NRSE measurement configuration. This configuration allows for a remote sense for the negative (–) input of the programmable gain instrumentation amplifier (PGIA) that is shared among all channels configured for NRSE mode. The behavior of this configuration is similar to the behavior of RSE connections but it provides improved noise rejection. Refer to Figure 6-3 for an illustration of an NRSE connection configuration.

AI1

AI0

AISENSE

AI GND

Figure 6-3. Connecting a Device to a USB-6215/6218 Using NRSE Connections

MUX

PGIA

ADC

USB-6215/6218

In the NRSE connection configuration, each input channel is measured with respect to AI SENSE.

NI USB-621x User Manual 6-4 ni.com

Analog Output

Many M Series devices have analog output functionality. NI 621x devices that support analog output have two AO channels controlled by a single clock and capable of waveform generation. Refer to Appendix A,

Device-Specific Information, for information about the capabilities of your

device.

Figure 7-1 shows the analog output circuitry of M Series devices.

AO 0

DAC0

AO FIFO

Isolation

Barrier

(USB-6215

and USB-6218

devices only)

Digital

Isolators

AO Data

AO 1

DAC1

AO Sample Clock

Figure 7-1. M Series Analog Output Circuitry

Analog Output Circuitry

DACs

Digital-to-analog converters (DACs) convert digital codes to analog voltages.

AO FIFO

The AO FIFO enables analog output waveform generation. It is a first-in-first-out (FIFO) memory buffer between the computer and the

Chapter 7 Analog Output

DACs. It allows you to download the points of a waveform to your M Series device without host computer interaction.

AO Sample Clock

The AO Sample Clock signal reads a sample from the DAC FIFO and generates the AO voltage.

AO Range

The AO Range is ±10 V for NI 621x devices.

Minimizing Glitches on the Output Signal

When you use a DAC to generate a waveform, you may observe glitches on the output signal. These glitches are normal; when a DAC switches from one voltage to another, it produces glitches due to released charges. The largest glitches occur when the most significant bit of the DAC code changes. You can build a lowpass deglitching filter to remove some of these glitches, depending on the frequency and nature of the output signal. Visit

ni.com/support for more information about minimizing glitches.

Analog Output Data Generation Methods

When performing an analog output operation, you either can perform software-timed or hardware-timed generations.

Software-Timed Generations

With a software-timed generation, software controls the rate at which data is generated. Software sends a separate command to the hardware to initiate each DAC conversion. In NI-DAQmx, software-timed generations are referred to as on-demand timing. Software-timed generations are also referred to as immediate or static operations. They are typically used for writing a single value out, such as a constant DC voltage.

Hardware-Timed Generations

With a hardware-timed generation, a digital hardware signal controls the rate of the generation. This signal can be generated internally on your device or provided externally.

NI USB-621x User Manual 7-2 ni.com

Chapter 7 Analog Output

Hardware-timed generations have several advantages over software-timed acquisitions:

• The time between samples can be much shorter.

• The timing between samples can be deterministic.

• Hardware-timed acquisitions can use hardware triggering.

Hardware-timed operations can be buffered or non-buffered.

During hardware-timed AO generation, data is moved from a PC buffer to the onboard FIFO on the DAQ device using USB Signal Streams before it is written to the DACs one sample at a time. Buffered acquisitions allow for fast transfer rates because data is moved in large blocks rather than one point at a time.

One property of buffered I/O operations is the sample mode. The sample mode can be either finite or continuous.

Finite sample mode generation refers to the generation of a specific, predetermined number of data samples. Once the specified number of samples has been written out, the generation stops.

Continuous generation refers to the generation of an unspecified number of samples. Instead of generating a set number of data samples and stopping, a continuous generation continues until you stop the operation. There are several different methods of continuous generation that control what data is written. These methods are regeneration, FIFO regeneration and non-regeneration modes.

Regeneration is the repetition of the data that is already in the buffer. Standard regeneration is when data from the PC buffer is continually downloaded to the FIFO to be written out. New data can be written to the PC buffer at any time without disrupting the output.

With FIFO regeneration, the entire buffer is downloaded to the FIFO and regenerated from there. Once the data is downloaded, new data cannot be written to the FIFO. To use FIFO regeneration, the entire buffer must fit within the FIFO size. The advantage of using FIFO regeneration is that it does not require communication with the main host memory once the operation is started, thereby preventing any problems that may occur due to excessive bus traffic.

With non-regeneration, old data will not be repeated. New data must be continually written to the buffer. If the program does not write new data to

Chapter 7 Analog Output

the buffer at a fast enough rate to keep up with the generation, the buffer will underflow and cause an error.

Analog Output Digital Triggering

Analog output supports two different triggering actions:

• Start trigger

• Pause trigger

A digital trigger can initiate these actions on the USB-621x devices. Refer to the AO Start Trigger Signal and AO Pause Trigger Signal sections for more information about these triggering actions.

Connecting Analog Output Signals

AO <0..1> are the voltage output signals for AO channels 0 and 1. AO GND is the ground reference for AO <0..1>.

Figure 7-2 shows how to make AO connections to the device.

Isolation

Barrier

(USB-6215

Analog Output Channels

AO 0

Load

NI USB-621x User Manual 7-4 ni.com

V OUT

–

AO GND

–

AO 1

Figure 7-2. Analog Output Connections

Channel 0

Channel 1

M Series Device

and USB-6218

devices only)

Digital

Isolators

Analog Output Timing Signals

Figure 7-3 summarizes all of the timing options provided by the analog output timing engine.

Chapter 7 Analog Output

PFI

20 MHz Timebase

100 kHz Timebase

USB M Series devices feature the following AO (waveform generation) timing signals.

• AO Start Trigger Signal

• AO Pause Trigger Signal

• AO Sample Clock Signal

• AO Sample Clock Timebase Signal

AO Start Trigger Signal

Use the AO Start Trigger (ao/StartTrigger) signal to initiate a waveform generation. If you do not use triggers, you can begin a generation with a software command.

ao/SampleClock

Timebase

Figure 7-3. Analog Output Timing Options

Ctr

Internal Output

Programmable

Clock

Divider

SampleClock

Timebase Divisor

ao/SampleClock

Using a Digital Source

To use ao/StartTrigger, specify a source and an edge. The source can be one of the following signals:

•A software pulse

• PFI <0..3>, PFI <8..11>

• ai/StartTrigger

Chapter 7 Analog Output

The source also can be one of several internal signals on your DAQ device. Refer to Device Routing in MAX in the NI-DAQmx Help or the LabVIEW 8.x Help for more information.

You also can specify whether the waveform generation begins on the rising edge or falling edge of ao/StartTrigger.

Routing AO Start Trigger Signal to an Output Terminal

You can route ao/StartTrigger out to any PFI <4..7> or PFI <12..15> terminal.

The output is an active high pulse.

AO Pause Trigger Signal

Use the AO Pause Trigger signal (ao/PauseTrigger) to mask off samples in a DAQ sequence. That is, when ao/PauseTrigger is active, no samples occur.

ao/PauseTrigger does not stop a sample that is in progress. The pause does not take effect until the beginning of the next sample.

If you are using any signal other than the onboard clock as the source of your sample clock, the generation resumes as soon as the pause trigger is deasserted and another edge of the sample clock is received, as shown in Figure 7-4.

Pause Trigger

Sample Clock

Figure 7-4. ao/PauseTrigger with Other Signal Source

Using a Digital Source

To use ao/PauseTrigger, specify a source and a polarity. The source can be the PFI <0..3> or PFI <8..11> signals.

NI USB-621x User Manual 7-6 ni.com

The source also can be one of several other internal signals on your DAQ device. Refer to Device Routing in MAX in the NI-DAQmx Help or the LabVIEW 8.x Help for more information.

You also can specify whether the samples are paused when ao/PauseTrigger is at a logic high or low level.

AO Sample Clock Signal

Use the AO Sample Clock (ao/SampleClock) signal to initiate AO samples. Each sample updates the outputs of all of the DACs. You can specify an internal or external source for ao/SampleClock. You also can specify whether the DAC update begins on the rising edge or falling edge of ao/SampleClock.

Using an Internal Source

One of the following internal signals can drive ao/SampleClock.

• AO Sample Clock Timebase (divided down)

• Counter n Internal Output

A programmable internal counter divides down the AO Sample Clock Timebase signal.

Chapter 7 Analog Output

Using an External Source

Use the external signals PFI <0..3> or PFI <8..11> as the source of ao/SampleClock.

Routing AO Sample Clock Signal to an Output Terminal

You can route ao/SampleClock (as an active low signal) out to any PFI <4..7> or PFI <12..15> terminal.

Other Timing Requirements

A counter on your device internally generates ao/SampleClock unless you select some external source. ao/StartTrigger starts the counter and either the software or hardware can stop it once a finite generation completes. When using an internally generated ao/SampleClock, you also can specify a configurable delay from ao/StartTrigger to the first ao/SampleClock pulse. By default, this delay is two ticks of ao/SampleClockTimebase.

Chapter 7 Analog Output

Figure 7-5 shows the relationship of ao/SampleClock to ao/StartTrigger.

ao/SampleClockTimebase

ao/SampleClock

Figure 7-5. ao/SampleClock and ao/StartTrigger

AO Sample Clock Timebase Signal

The AO Sample Clock Timebase (ao/SampleClockTimebase) signal is divided down to provide a source for ao/SampleClock.

You can route any of the following signals to be the AO Sample Clock Timebase (ao/SampleClockTimebase) signal:

•20MHzTimebase

• 100 kHz Timebase

• PFI <0..3>, PFI <8..11>

ao/StartTrigger

Delay

From

Start

Trigger

ao/SampleClockTimebase is not available as an output on the I/O connector.

You might use ao/SampleClockTimebase if you want to use an external sample clock signal, but need to divide the signal down. If you want to use an external sample clock signal, but do not need to divide the signal, then you should use ao/SampleClock rather than ao/SampleClockTimebase.

NI USB-621x User Manual 7-8 ni.com

Chapter 7 Analog Output

Getting Started with AO Applications in Software

You can use an NI 621x device in the following analog output applications.

• Single-point (on-demand) generation

• Finite generation

• Continuous generation

• Waveform generation

You can perform these generations through programmed I/O or USB Signal Stream data transfer mechanisms. Some of the applications also use start triggers and pause triggers.

Note For more information about programming analog output applications and triggers in software, refer to the NI-DAQmx Help or the LabVIEW 8.x Help.

Digital I/O

NI 621x devices have eight static digital input lines, P0.<0..7>. These lines also can be used as PFI inputs.

NI 621x devices have eight static digital output lines, P1.<0..8>.These lines also can be used as PFI output. By default the digital output lines are disabled (high impedance with a 47 kΩ pull down resistor) on power up. Software can enable or disable the entire port (software cannot enable individual lines). Once the port is enabled, you can individually configure each line to the following:

• Set a line to a static 0

• Set a line to a static 1

• Export a timing output signal to a line as a PFI pin

The voltage input and output levels and the current drive level of the DI and DO lines are listed in the NI 621x Specifications. Refer to Chapter 10, PFI, for more information on PFI inputs and outputs.

Figure 8-1 shows the circuitry of one DI line and one DO line. The following sections provide information about the various parts of the DIO circuit.

Static DI

Static DO

Figure 8-1. M Series Digital I/O Circuitry

The DI terminals are named P0.<0..7> on the NI 621x device I/O connector. The DO terminals are named P1.<0..7> on the NI 621x device I/O connector.

I/O Protection

47kΩ Pull-Down

I/O Protection

47 kΩ Pull-Down

P0.

P1.

Chapter 8 Digital I/O

Static DIO

I/O Protection

The voltage input and output levels and the current drive levels of the DIO lines are listed in the specifications of your device.

You can use static DI and DO lines to monitor or control digital signals.

All samples of static DI lines and updates of DO lines are software-timed.

Each DI, DO, and PFI signal is protected against overvoltage, undervoltage, and overcurrent conditions as well as ESD events. However, you should avoid these fault conditions by following these guidelines.

•Do not connect a DO or PFI output lines to any external signal source, ground signal, or power supply.

• Understand the current requirements of the load connected to DO or PFI output signals. Do not exceed the specified current output limits of the DAQ device. NI has several signal conditioning solutions for digital applications requiring high current drive.

•Do not drive a DI or PFI input line with voltages outside of its normal operating range. The PFI or DI lines have a smaller operating range than the AI signals.

Increasing Current Drive

The total internal current limit for digital outputs and power down from the +5 V terminals is 50 mA. You can increase this internal current limit by supplying an external +5 V supply. Refer to the +5 V Power as an Input section of Chapter 3, Connector Information.

NI USB-621x User Manual 8-2 ni.com

Connecting Digital I/O Signals

The DI and DO signals, P0.<0..7> and P1.<0..7> are referenced to D GND. Digital input applications include receiving TTL signals and sensing external device states, such as the state of the switch shown in the figure. Digital output applications include sending TTL signals and driving external devices, such as the LED shown in Figure 8-2.

Chapter 8 Digital I/O

+5 V

LED

+5 V

Switch

P1.<0..3>

TTL Signal

D GND

I/O Connector

When using a USB-6215/6218, you must connect D GND

and/or AI GND to the local ground on your system.

M Series Device

P0.<0..3>

Isolation

Barrier

(USB-6215

and USB-6218

devices only)

Digital

Isolators

Figure 8-2. Digital I/O Connections

Caution Exceeding the maximum input voltage ratings, which are listed in the

specifications document for each M Series device, can damage the DAQ device and the computer. NI is not liable for any damage resulting from such signal connections.

Chapter 8 Digital I/O

Getting Started with DIO Applications in Software

You can use the M Series device in the following digital I/O applications:

• Static digital input

• Static digital output

• Digital waveform generation

• Digital waveform acquisition

• DI change detection

Note For more information about programming digital I/O applications and triggers in software, refer to the NI-DAQmx Help or the LabVIEW 8.x Help.

NI USB-621x User Manual 8-4 ni.com

Counters

M Series devices have two general-purpose 32-bit counter/timers and one frequency generator, as shown in Figure 9-1. The general-purpose counter/timers can be used for many measurement and pulse generation applications.

Input Selection Muxes

Counter 0

Counter 0 Source (Counter 0 Timebase)

Counter 0 Gate

Counter 0 Aux

Counter 0 HW Arm

Counter 0 A

Counter 0 B (Counter 0 Up_Down)

Counter 0 Z

Counter 1 Source (Counter 1 Timebase)

Counter 1 Gate

Counter 1 Aux

Counter 1 HW Arm

Counter 1 A

Counter 1 B (Counter 1 Up_Down)

Counter 1 Z

Counter 0 Internal Output

Counter 1

Counter 0 Internal Output

Counter 0 TC

Input Selection Muxes

Frequency Output Timebase Freq Out

Frequency Generator

Figure 9-1. M Series Counters

Chapter 9 Counters

The counters have seven input signals, although in most applications only a few inputs are used.

For information about connecting counter signals, refer to the Default

Counter/Timer Pinouts section.

Counter Input Applications

Counting Edges

In edge counting applications, the counter counts edges on its Source after the counter is armed. You can configure the counter to count rising or falling edges on its Source input. You also can control the direction of counting (up or down).

The counter values can be read on demand or with a sample clock.

Single Point (On-Demand) Edge Counting

With single point (on-demand) edge counting, the counter counts the number of edges on the Source input after the counter is armed. On-demand refers to the fact that software can read the counter contents at any time without disturbing the counting process. Figure 9-2 shows an example of single point edge counting.

Counter Armed

SOURCE

Counter Value 105432

Figure 9-2. Single Point (On-Demand) Edge Counting

You also can use a pause trigger to pause (or gate) the counter. When the pause trigger is active, the counter ignores edges on its Source input. When the pause trigger is inactive, the counter counts edges normally.

You can route the pause trigger to the Gate input of the counter. You can configure the counter to pause counting when the pause trigger is high or when it is low. Figure 9-3 shows an example of on-demand edge counting with a pause trigger.

NI USB-621x User Manual 9-2 ni.com

Pause Trigger

(Pause When Low)

SOURCE

Chapter 9 Counters

Counter Armed

Counter Value

100 5432

Figure 9-3. Single Point (On-Demand) Edge Counting with Pause Trigger

Buffered (Sample Clock) Edge Counting

With buffered edge counting (edge counting using a sample clock), the counter counts the number of edges on the Source input after the counter is armed. The value of the counter is sampled on each active edge of a sample clock. A USB Signal Stream transfers the sampled values to host memory.

The count values returned are the cumulative counts since the counter armed event. That is, the sample clock does not reset the counter.

You can route the counter sample clock to the Gate input of the counter. You can configure the counter to sample on the rising or falling edge of the sample clock.

Figure 9-4 shows an example of buffered edge counting. Notice that counting begins when the counter is armed, which occurs before the first active edge on Gate.

Counter Armed

(Sample on Rising Edge)

Sample Clock

SOURCE

Counter Value

Buffer

10763452

3 6

Figure 9-4. Buffered (Sample Clock) Edge Counting

Chapter 9 Counters

Non-Cumulative Buffered Edge Counting

Non-cumulative edge counting is similar to buffered (sample clock) edge counting. However, the counter resets after each active edge of the Sample Clock. You can route the Sample Clock to the Gate input of the counter.

Figure 9-5 shows an example of non-cumulative buffered edge counting.

Counter

Sample Clock

(Sample on

Rising Edge)

SOURCE

Armed

Counter Value

Buffer

Figure 9-5. Non-Cumulative Buffered Edge Counting

1101331222

2232

3 3

Notice that the first count interval begins when the counter is armed, which occurs before the first active edge on Gate.

Note that if you are using an external signal as the Source, at least one Source pulse should occur between each active edge of the Gate signal. This condition ensures that correct values are returned by the counter. If this condition is not met, consider using duplicate count prevention, described in the Duplicate Count Prevention section.

Controlling the Direction of Counting

In edge counting applications, the counter can count up or down. You can configure the counter to do the following:

• Always count up

• Always count down

• Count up when the Counter n B input is high; count down when it is low

For information about connecting counter signals, refer to the Default

Counter/Timer Pinouts section.

NI USB-621x User Manual 9-4 ni.com

Pulse-Width Measurement

In pulse-width measurements, the counter measures the width of a pulse on its Gate input signal. You can configure the counter to measure the width of high pulses or low pulses on the Gate signal.

You can route an internal or external periodic clock signal (with a known period) to the Source input of the counter. The counter counts the number of rising (or falling) edges on the Source signal while the pulse on the Gate signal is active.

You can calculate the pulse width by multiplying the period of the Source signal by the number of edges returned by the counter.

A pulse-width measurement will be accurate even if the counter is armed while a pulse train is in progress. If a counter is armed while the pulse is in the active state, it will wait for the next transition to the active state to begin the measurement.

Single Pulse-Width Measurement

With single pulse-width measurement, the counter counts the number of edges on the Source input while the Gate input remains active. When the Gate input goes inactive, the counter stores the count in a hardware save register and ignores other edges on the Gate and Source inputs. Software then reads the stored count.

Chapter 9 Counters

Figure 9-6 shows an example of a single pulse-width measurement.

GATE

SOURCE

Counter Value

HW Save Register

Figure 9-6. Single Pulse-Width Measurement

Buffered Pulse-Width Measurement

Buffered pulse-width measurement is similar to single pulse-width measurement, but buffered pulse-width measurement takes measurements over multiple pulses.

Chapter 9 Counters

SOURCE

The counter counts the number of edges on the Source input while the Gate input remains active. On each trailing edge of the Gate signal, the counter stores the count in a hardware save register. A USB Signal Stream transfers the stored values to host memory.

Figure 9-7 shows an example of a buffered pulse-width measurement.

GATE

COUNTER VALUE

BUFFER

For information about connecting counter signals, refer to the Default

Counter/Timer Pinouts section.

Period Measurement

In period measurements, the counter measures a period on its Gate input signal after the counter is armed. You can configure the counter to measure the period between two rising edges or two falling edges of the Gate input signal.

You can route an internal or external periodic clock signal (with a known period) to the Source input of the counter. The counter counts the number of rising (or falling) edges occurring on the Source input between the two active edges of the Gate signal.

1 0 3

Figure 9-7. Buffered Pulse-Width Measurement

3 3

2 1 2

You can calculate the period of the Gate input by multiplying the period of the Source signal by the number of edges returned by the counter.

NI USB-621x User Manual 9-6 ni.com

Chapter 9 Counters

Single Period Measurement

With single period measurement, the counter counts the number of rising (or falling) edges on the Source input occurring between two active edges of the Gate input. On the second active edge of the Gate input, the counter stores the count in a hardware save register and ignores other edges on the Gate and Source inputs. Software then reads the stored count.

Figure 9-8 shows an example of a single period measurement.

GATE

SOURCE

Counter Value

HW Save Register

103

Figure 9-8. Single Period Measurement

Buffered Period Measurement

Buffered period measurement is similar to single period measurement, but buffered period measurement measures multiple periods.

The counter counts the number of rising (or falling) edges on the Source input between each pair of active edges on the Gate input. At the end of each period on the Gate signal, the counter stores the count in a hardware save register. A USB Signal Stream transfers the stored values to host memory.

The counter begins on the first active edge of the Gate after it is armed. The arm usually occurs in the middle of a period of the Gate input. The counter does not store a measurement for this incomplete period.

Figure 9-9 shows an example of a buffered period measurement. In this example, a period is defined by two consecutive rising edges.

Chapter 9 Counters

SOURCE

Counter Value

GATE

Buffer

ounter Armed

311

3 3

Time N

Figure 9-9. Buffered Period Measurement

Table 9-1. Time N Descriptions

At t0, the counter is armed. No measurements are taken until the counter is armed.

The rising edge of Gate indicates the beginning of the first period to measure. The counter begins counting rising edges of Source.

The rising edge of Gate indicates the end of the first period. The USB M Series device stores the counter value in the buffer.

The rising edge of Gate indicates the end of the second period. The USB M Series device stores the counter value in the buffer.

Count Prevention section for more information.

For information about connecting counter signals, refer to the Default

Counter/Timer Pinouts section.

NI USB-621x User Manual 9-8 ni.com

Semi-Period Measurement

In semi-period measurements, the counter measures a semi-period on its Gate input signal after the counter is armed. A semi-period is the time between any two consecutive edges on the Gate input.

You can calculate the semi-period of the Gate input by multiplying the period of the Source signal by the number of edges returned by the counter.

Single Semi-Period Measurement

Single semi-period measurement is equivalent to single pulse-width measurement.

Buffered Semi-Period Measurement

In buffered semi-period measurement, on each edge of the Gate signal, the counter stores the count in a hardware save register. A USB Signal Stream transfers the stored values to host memory.

Chapter 9 Counters

The counter begins counting on the first active edge of the Gate after it is armed. The arm usually occurs between edges on the Gate input. The counter does not store a value for this incomplete semi-period.

Figure 9-10 shows an example of a buffered semi-period measurement.

Counter Armed

GATE

SOURCE

1 1

Counter Value

Buffer

Figure 9-10. Buffered Semi-Period Measurement

1 3

1 2

1 3

Note that if you are using an external signal as the Source, at least one Source pulse should occur between each active edge of the Gate signal.

Chapter 9 Counters

This condition ensures that correct values are returned by the counter. If this condition is not met, the counter returns a zero. Refer to the Duplicate

Count Prevention section for more information.

For information about connecting counter signals, refer to the Default

Counter/Timer Pinouts section.

Frequency Measurement

You can use the counters to measure frequency in several different ways. You can choose one of the following methods depending on your application.

Method 1—Measure Low Frequency with One Counter

In this method, you measure one period of your signal using a known timebase. This method is good for low frequency signals.

You can route the signal to measure (F1) to the Gate of a counter. You can route a known timebase (Ft) to the Source of the counter. The known timebase can be 80MHzTimebase. For signals that might be slower than

0.02 Hz, use a slower known timebase.

You can configure the counter to measure one period of the gate signal. The frequency of F1 is the inverse of the period. Figure 9-11 illustrates this method.

Interval Measured

Gate

Source

Single Period

Measurement

NI USB-621x User Manual 9-10 ni.com

1 2 3 …

Figure 9-11. Method 1

Period of F1 =

Frequency of F1 =

…

Chapter 9 Counters

Method 1b—Measure Low Frequency with One Counter (Averaged)

In this method, you measure several periods of your signal using a known timebase. This method is good for low to medium frequency signals.

0.02 Hz, use a slower known timebase.

You can configure the counter to make K + 1 buffered period measurements. Recall that the first period measurement in the buffer should be discarded.

Average the remaining K period measurements to determine the average period of F1. The frequency of F1 is the inverse of the average period. Figure 9-12 illustrates this method.

Intervals Measured

…T

Gate

Source

Buffered Period

Measurement

2 ...

1... ...

Average Period of F1 =

Frequency of F1 =

… 1... ...

K × Ft

+ …

Figure 9-12. Method 1b

Method 2—Measure High Frequency with Two Counters

In this method, you measure one pulse of a known width using your signal and derive the frequency of your signal from the result. This method is good for high frequency signals.

In this method, you route a pulse of known duration (T) to the Gate of a counter. You can generate the pulse using a second counter. You also can

Chapter 9 Counters

generate the pulse externally and connect it to a PFI terminal. You only need to use one counter if you generate the pulse externally.

Route the signal to measure (F1) to the Source of the counter. Configure the counter for a single pulse-width measurement. Suppose you measure the width of pulse T to be N periods of F1. Then the frequency of F1 is N/T.

Figure 9-13 illustrates this method. Another option would be to measure the width of a known period instead of a known pulse.

Width of Pulse (T)

Pulse

Gate

Source

Pulse-Width

Measurement

12…

Width of

Frequency of F1 =

Figure 9-13. Method 2

Pulse

T =

Method 3—Measure Large Range of Frequencies Using Two Counters

By using two counters, you can accurately measure a signal that might be high or low frequency. This technique is called reciprocal frequency measurement. In this method, you generate a long pulse using the signal to measure. You then measure the long pulse with a known timebase. The M Series device can measure this long pulse more accurately than the faster input signal.

You can route the signal to measure to the Source input of Counter 0, as shown in Figure 9-14. Assume this signal to measure has frequency F1. Configure Counter 0 to generate a single pulse that is the width of N periods of the source input signal.

NI USB-621x User Manual 9-12 ni.com

Chapter 9 Counters

Signal to

Measure (F1)

Signal of Known

Frequency (F2)

CTR_0_SOURCE

(Signal to Measure)

CTR_0_OUT

(CTR_1_GATE)

CTR_1_SOURCE

SOURCE OUT

COUNTER 0

SOURCE

COUNTER 1

GATE

0 1 2 3 …

Interval

to Measure

Figure 9-14. Method 3

OUT

Then route the Counter 0 Internal Output signal to the Gate input of Counter 1. You can route a signal of known frequency (F2) to the Counter 1 Source input. F2 can be 80MHzTimebase. For signals that might be slower than 0.02 Hz, use a slower known timebase. Configure Counter 1 to perform a single pulse-width measurement. Suppose the result is that the pulse width is J periods of the F2 clock.

From Counter 0, the length of the pulse is N/F1. From Counter 1, the length of the same pulse is J/F2. Therefore, the frequency of F1 is given by F1=F2*(N/J).

Choosing a Method for Measuring Frequency

The best method to measure frequency depends on several factors including the expected frequency of the signal to measures, the desired accuracy, how many counters are available, and how long the measurement can take.

• Method 1 uses only one counter. It is a good method for many applications. However, the accuracy of the measurement decreases as the frequency increases.

Consider a frequency measurement on a 50 kHz signal using an 80 MHz Timebase. This frequency corresponds to 1600 cycles of the

Chapter 9 Counters

80 MHz Timebase. Your measurement may return 1600 ± 1 cycles depending on the phase of the signal with respect to the timebase. As your frequency becomes larger, this error of ±1 cycle becomes more significant; Table 9-2 illustrates this point.

Table 9-2. Frequency Measurement Method 1

Ta sk Equation Example 1 Example 2

Actual Frequency to Measure F1 50 kHz 5MHz

Timebase Frequency Ft 80 MHz 80 MHz

Actual Number of Timebase

Ft/F1 1600 16

Periods

Worst Case Measured Number of

(Ft/F1) – 1 1599 15

Timebase Periods

Measured Frequency Ft F1/(Ft – F1) 50.125 kHz 5.33 MHz

Error [Ft F1/(Ft – F1)] – F1 125 kHz 333 kHz

Error % [Ft/(Ft – F1)] – 1 0.06% 6.67%

• Method 1b (measuring K periods of F1) improves the accuracy of the measurement. A disadvantage of Method 1b is that you have to take K + 1 measurements. These measurements take more time and consume some of the available USB bandwidth.

• Method 2 is accurate for high frequency signals. However, the accuracy decreases as the frequency of the signal to measure decreases. At very low frequencies, Method 2 may be too inaccurate for your application. Another disadvantage of Method 2 is that it requires two counters (if you cannot provide an external signal of known width). An advantage of Method 2 is that the measurement completes in a known amount of time.

• Method 3 measures high and low frequency signals accurately. However, it requires two counters.

NI USB-621x User Manual 9-14 ni.com

Chapter 9 Counters

Table 9-3 summarizes some of the differences in methods of measuring frequency.

Table 9-3. Frequency Measurement Method Comparison

Number of

Method

1 1 1 Poor Good

1b 1 Many Fair Good

2 1 or 2 1 Good Poor

3 2 1 Good Good

Counters Used

For information about connecting counter signals, refer to the Default

Counter/Timer Pinouts section.

Position Measurement

You can use the counters to perform position measurements with quadrature encoders or two-pulse encoders. You can measure angular position with X1, X2, and X4 angular encoders. Linear position can be measured with two-pulse encoders. You can choose to do either a single point (on-demand) position measurement or a buffered (sample clock) position measurement. You must arm a counter to begin position measurements.

Number of

Measurements

Returned

Measures High

Frequency

Signals

Accurately

Measures Low

Frequency

Signals

Accurately

Measurements Using Quadrature Encoders

The counters can perform measurements of quadrature encoders that use X1, X2, or X4 encoding. A quadrature encoder can have up to three channels—channels A, B, and Z.

X1 Encoding

When channel A leads channel B in a quadrature cycle, the counter increments. When channel B leads channel A in a quadrature cycle, the counter decrements. The amount of increments and decrements per cycle depends on the type of encoding—X1, X2, or X4.

Chapter 9 Counters

Figure 9-15 shows a quadrature cycle and the resulting increments and decrements for X1 encoding. When channel A leads channel B, the increment occurs on the rising edge of channel A. When channel B leads channel A, the decrement occurs on the falling edge of channel A.

Ch A Ch B

Counter Value

Figure 9-15. X1 Encoding

X2 Encoding

The same behavior holds for X2 encoding except the counter increments or decrements on each edge of channel A, depending on which channel leads the other. Each cycle results in two increments or decrements, as shown in Figure 9-16.

Ch A Ch B

Counter Value

56 8

Figure 9-16. X2 Encoding

X4 Encoding

Similarly, the counter increments or decrements on each edge of channels A and B for X4 encoding. Whether the counter increments or decrements depends on which channel leads the other. Each cycle results in four increments or decrements, as shown in Figure 9-17.

Ch A

Ch B

Counter Value 5 6 8 9 10 1011 1112 1213 137

56879

Figure 9-17. X4 Encoding

NI USB-621x User Manual 9-16 ni.com

Chapter 9 Counters

Channel Z Behavior

Some quadrature encoders have a third channel, channel Z, which is also referred to as the index channel. A high level on channel Z causes the counter to be reloaded with a specified value in a specified phase of the quadrature cycle. You can program this reload to occur in any one of the four phases in a quadrature cycle.

Channel Z behavior—when it goes high and how long it stays high—differs with quadrature encoder designs. You must refer to the documentation for your quadrature encoder to obtain timing of channel Z with respect to channels A and B. You must then ensure that channel Z is high during at least a portion of the phase you specify for reload. For instance, in Figure 9-18, channel Z is never high when channel A is high and channel B is low. Thus, the reload must occur in some other phase.

In Figure 9-18, the reload phase is when both channel A and channel B are low. The reload occurs when this phase is true and channel Z is high. Incrementing and decrementing takes priority over reloading. Thus, when the channel B goes low to enter the reload phase, the increment occurs first. The reload occurs within one maximum timebase period after the reload phase becomes true. After the reload occurs, the counter continues to count as before. The figure illustrates channel Z reload with X4 decoding.

Ch A Ch B

Ch Z

Max Timebase

Counter Value

Figure 9-18. Channel Z Reload with X4 Decoding

890 21743

A = 0 B = 0 Z = 1

Measurements Using Two Pulse Encoders

The counter supports two pulse encoders that have two channels—channels A and B.

Chapter 9 Counters

The counter increments on each rising edge of channel A. The counter decrements on each rising edge of channel B, as shown in Figure 9-19.

Ch A

Ch B

Counter Value 2 3 54344

Figure 9-19. Measurements Using Two Pulse Encoders

For information about connecting counter signals, refer to the Default

Counter/Timer Pinouts section.

Two-Signal Edge-Separation Measurement

Two-signal edge-separation measurement is similar to pulse-width measurement, except that there are two measurement signals—Aux and Gate. An active edge on the Aux input starts the counting and an active edge on the Gate input stops the counting. You must arm a counter to begin a two edge separation measurement.

After the counter has been armed and an active edge occurs on the Aux input, the counter counts the number of rising (or falling) edges on the Source. The counter ignores additional edges on the Aux input.

The counter stops counting upon receiving an active edge on the Gate input. The counter stores the count in a hardware save register.

You can configure the rising or falling edge of the Aux input to be the active edge. You can configure the rising or falling edge of the Gate input to be the active edge.

Use this type of measurement to count events or measure the time that occurs between edges on two signals. This type of measurement is sometimes referred to as start/stop trigger measurement, second gate measurement, or A-to-B measurement.

Single Two-Signal Edge-Separation Measurement

With single two-signal edge-separation measurement, the counter counts the number of rising (or falling) edges on the Source input occurring between an active edge of the Gate signal and an active edge of the Aux signal. The counter then stores the count in a hardware save register and ignores other edges on its inputs. Software then reads the stored count.

NI USB-621x User Manual 9-18 ni.com

Chapter 9 Counters

Figure 9-20 shows an example of a single two-signal edge-separation measurement.

Counter

AUX

GATE

SOURCE

Counter Value

HW Save Register

Armed

0000123456788 8

Measured Interval

Figure 9-20. Single Two-Signal Edge-Separation Measurement

Buffered Two-Signal Edge-Separation Measurement

Buffered and single two-signal edge-separation measurements are similar, but buffered measurement measures multiple intervals.

The counter counts the number of rising (or falling) edges on the Source input occurring between an active edge of the Gate signal and an active edge of the Aux signal. The counter then stores the count in a hardware save register. On the next active edge of the Gate signal, the counter begins another measurement. A USB Signal Stream transfers the stored values to host memory.

Figure 9-21 shows an example of a buffered two-signal edge-separation measurement.

AUX

GATE

SOURCE

Counter Value

Buffer

123 123 123

3333

3 3

Figure 9-21. Buffered Two-Signal Edge-Separation Measurement

For information about connecting counter signals, refer to the Default

Counter/Timer Pinouts section.

Chapter 9 Counters

Counter Output Applications

Simple Pulse Generation

Single Pulse Generation

The counter can output a single pulse. The pulse appears on the Counter n Internal Output signal of the counter.

You can specify a delay from when the counter is armed to the beginning of the pulse. The delay is measured in terms of a number of active edges of the Source input.

You can specify a pulse width. The pulse width is also measured in terms of a number of active edges of the Source input. You also can specify the active edge of the Source input (rising or falling).

Figure 9-22 shows a generation of a pulse with a pulse delay of four and a pulse width of three (using the rising edge of Source).

Counter Armed

SOURCE

OUT

Figure 9-22. Single Pulse Generation

Single Pulse Generation with Start Trigger

The counter can output a single pulse in response to one pulse on a hardware Start Trigger signal. The pulse appears on the Counter n Internal Output signal of the counter.

You can route the Start Trigger signal to the Gate input of the counter. You can specify a delay from the Start Trigger to the beginning of the pulse. You also can specify the pulse width. The delay and pulse width are measured in terms of a number of active edges of the Source input.

After the Start Trigger signal pulses once, the counter ignores the Gate input.

NI USB-621x User Manual 9-20 ni.com

National Instruments NI USB-621x User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual

NI USB-621x User Manual

Support

Worldwide Technical Support and Product Information

National Instruments Corporate Headquarters

Worldwide Offices

Important Information

Warranty

Copyright

Trademarks

Patents

WARNING REGARDING USE OF NATIONAL INSTRUMENTS PRODUCTS

Contents

About This Manual

Conventions

Related Documentation

NI-DAQmx for Windows

LabVIEW

Measurement Studio

ANSI C without NI Application Software

.NET Languages without NI Application Software

Device Documentation and Specifications

Training Courses

Technical Support on the Web

Figure 1-1. USB-6210/6211

Figure 1-2. USB-6215/6218

Installing NI-DAQmx

Installing Other Software

Installing the Hardware

Device Pinouts

Device Specifications

Device Accessories

DAQ Hardware

Figure 2-1. Components of a Typical DAQ System

DAQ-STC2

Figure 2-2. USB-621x Block Diagram

Calibration Circuitry

Signal Conditioning

Sensors and Transducers

Programming Devices in Software

I/O Connector Signal Descriptions

Table 3-1. I/O Connector Signals

+5 V Power

+5 V Power as an Output

+5 V Power as an Input

Figure 4-1. M Series Analog Input Circuitry

Analog Input Circuitry

Analog Input Range

Table 4-1. Input Ranges for NI 621x

Analog Input Ground-Reference Settings

Table 4-2. Analog Input Ground-Reference Settings

Figure 4-2. NI-PGIA

Table 4-3. Signals Routed to the NI-PGIA

Configuring AI Ground-Reference Settings in Software

Figure 4-3. Enabling Multimode Scanning in LabVIEW

Multichannel Scanning Considerations

Use Low Impedance Sources

Carefully Choose the Channel Scanning Order

Avoid Switching from a Large to a Small Input Range

Insert Grounded Channel between Signal Channels

Minimize Voltage Step between Adjacent Channels

Avoid Scanning Faster Than Necessary

Example 1

Example 2

Analog Input Data Acquisition Methods

Software-Timed Acquisitions

Hardware-Timed Acquisitions

Buffered

Non-Buffered

Analog Input Digital Triggering

Field Wiring Considerations

Analog Input Timing Signals

Figure 4-4. Analog Input Timing Options

Figure 4-5. Interval Sampling

Figure 4-6. Posttriggered Data Acquisition Example

Figure 4-7. Pretriggered Data Acquisition Example

AI Sample Clock Signal