National Instruments NI-USB-621x User Manual

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DAQ M Series

NI USB-621x User Manual

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NI USB-621x User Manual

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March 2008 371931E-01

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Important Information

Warranty

The NI USB-6210, NI USB-6211, NI USB-6212, NI USB-6215, NI USB-6216, and NI USB-6218 are warranted against defects in materials and workmanship for a period of one year 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 du ring 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 instructions 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 consu Instruments be liable for any damages arising out of or related to this document or the information contained in it.

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lt National Instruments if errors are suspected. In no event shall National

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Compliance

Compliance with FCC/Canada Radio Frequency Interference Regulations

Determining FCC Class

The Federal Communications Commission (FCC) has rules to protect wireless communications from interference. The FCC places digital electronics into two classes. These classes are known as Class A (for use in industrial-commercial locations only) or Class B (for use in residential or commercial locations). All National Instruments (NI) products are FCC Class A products.

Depending on where it is operated, this Class A product could be subject to restrictions in the FCC rules. (In Canada, the Department of Communications (DOC), of Industry Canada, regulates wireless interference in much the same way.) Digital electronics emit weak signals during normal operation that can affect radio, television, or other wireless products.

All Class A products display a simple warning statement of one paragraph in length regarding interference and undesired operation. The FCC rules have restrictions regarding the locations where FCC Class A products can be operated.

Consult the FCC Web site at

FCC/DOC Warnings

This equipment generates and uses radio frequency energy and, if not installed and used in strict accordance with the instructions in this manual and the CE marking Declaration of Conformity*, may cause interference to radio and television reception. Classification requirements are the same for the Federal Communications Commission (FCC) and the Canadian Department of Communications (DOC).

Changes or modifications not expressly approved by NI could void the user’s authority to operate the equipment under the FCC Rules.

Class A

Federal Communications Commission

This equipment has been tested and found to comply with the limits for a Class A digital device, pursuant to part 15 of the FCC Rules. These limits are designed to provide reasonable protection against harmful interference when the equipment is operated in a commercial environment. This equipment generates, uses, and can radiate radio frequency energy and, if not installed and used in accordance with the instruction manual, may cause harmful interference to radio communications. Operation of this equipment in a residential area is likely to cause harmful interference in which case the user is required to correct the interference at their own expense.

www.fcc.gov for more information.

Canadian Department of Communications

This Class A digital apparatus meets all requirements of the Canadian Interference-Causing Equipment Regulations. Cet appareil numérique de la classe A respecte toutes les exigences du Règlement sur le matériel brouilleur du Canada.

Compliance with EU Directives

Users in the European Union (EU) should refer to the Declaration of Conformity (DoC) for information* pertaining to the CE marking. Refer to the Declaration of Conformity (DoC) for this product for any additional regulatory compliance information. To obtain the DoC for this product, visit and click the appropriate link in the Certification column.

* The CE marking Declaration of Conformity contains important supplementary information and instructions for the user or

installer.

ni.com/certification, search by model number or product line,

About This Manual

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

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

Chapter 1 Getting Started

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

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

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

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

Device Specifications ....................................................................................................1-2

Applying Signal Labels to the USB-621x......................................................................1-2

USB Cable Strain Relief ................................................................................................1-3

Mounting the USB-621x ................................................................................................1-4

Desktop Use.....................................................................................................1-4

DIN Rail Mounting..........................................................................................1-4

Panel Mounting ...............................................................................................1-5

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

Cables and Accessories..................................................................................................2-4

USB-621x Mass Termination Custom Cabling...............................................2-4

Programming Devices in Software ................................................................................2-5

Chapter 3 Connector and LED Information

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

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

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

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

USB Device Fuse Replacement .....................................................................................3-3

PWR/ACT LED Indicator .............................................................................................3-5

Contents

Chapter 4 Analog Input

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

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

Multichannel Scanning Considerations......................................................................... 4-5

Analog Input Data Acquisition Methods....................................................................... 4-8

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

Field Wiring Considerations.......................................................................................... 4-10

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

Getting Started with AI Applications in Software ........................................................ 4-22

Connecting Analog Input Signals on USB-6210/6211/6212 Devices .......................... 4-23

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

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

Using an Internal Source .................................................................. 4-14

Using an External Source ................................................................. 4-14

Routing AI Sample Clock to an Output Terminal............................ 4-14

Other Timing Requirements ............................................................. 4-14

AI Sample Clock Timebase Signal ................................................................. 4-15

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

Using an Internal Source .................................................................. 4-16

Using an External Source ................................................................. 4-16

Routing AI Convert Clock to an Output Terminal ........................... 4-16

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

Other Timing Requirements ............................................................. 4-17

AI Convert Clock Timebase Signal ................................................................ 4-19

AI Hold Complete Event Signal ..................................................................... 4-19

AI Start Trigger Signal.................................................................................... 4-20

Using a Digital Source...................................................................... 4-20

Routing AI Start Trigger to an Output Terminal .............................. 4-20

AI Reference Trigger Signal ........................................................................... 4-21

Using a Digital Source...................................................................... 4-22

Routing AI Reference Trigger to an Output Terminal .....................4-22

AI Pause Trigger Signal.................................................................................. 4-22

Using a Digital Source...................................................................... 4-22

Connecting Floating Signal Sources ............................................................... 4-25

What Are Floating Signal Sources?.................................................. 4-25

When to Use Differential Connections with Floating

Signal Sources ............................................................................... 4-25

When to Use Referenced Single-Ended (RSE) Connections

with Floating Signal Sources ......................................................... 4-25

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

Connections with Floating Signal Sources .................................... 4-26

Using Differential Connections for Floating Signal Sources ...........4-27

NI USB-621x User Manual vi ni.com

Connecting Ground-Referenced Signal Sources .............................................4-31

Connecting Analog Input Signals on USB-6215/6216/6218 Devices ...........................4-36

Taking Differential Measurements..................................................................4-36

Taking Referenced Single-Ended (RSE) Measurements ................................4-37

Taking Non-Referenced Single-Ended (NRSE) Measurements .....................4-38

Chapter 5 Analog Output

AO Range ......................................................................................................................5-2

Minimizing Glitches on the Output Signal ....................................................................5-2

Analog Output Data Generation Methods .....................................................................5-2

Analog Output Digital Triggering .................................................................................5-3

Connecting Analog Output Signals ...............................................................................5-4

Analog Output Timing Signals ......................................................................................5-4

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

AO Pause Trigger Signal.................................................................................5-6

AO Sample Clock Signal.................................................................................5-7

AO Sample Clock Timebase Signal................................................................5-8

Getting Started with AO Applications in Software .......................................................5-9

Contents

Using Non-Referenced Single-Ended (NRSE) Connections

for Floating Signal Sources ............................................................4-30

Using Referenced Single-Ended (RSE) Connections for

Floating Signal Sources..................................................................4-31

What Are Ground-Referenced Signal Sources?................................4-31

When to Use Differential Connections with Ground-Referenced

Signal Sources................................................................................4-32

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

Connections with Ground-Referenced Signal Sources ..................4-32

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

Ground-Referenced Signal Sources ...............................................4-33

Using Differential Connections for Ground-Referenced

Signal Sources................................................................................4-33

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

Ground-Referenced Signal Sources ...............................................4-34

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

Routing AO Start Trigger to an Output Terminal .............................5-6

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

Using an Internal Source...................................................................5-7

Using an External Source..................................................................5-8

Routing AO Sample Clock to an Output Terminal...........................5-8

Other Timing Requirements..............................................................5-8

Contents

Chapter 6 Digital I/O

Digital I/O on USB-6210/6211/6215/6218 Devices ..................................................... 6-1

Digital I/O on USB-6212/6216 Devices........................................................................ 6-4

Chapter 7 PFI

Using PFI Terminals as Timing Input Signals .............................................................. 7-2

Exporting Timing Output Signals Using PFI Terminals............................................... 7-3

Using PFI Terminals as Static Digital I/Os ................................................................... 7-3

Connecting PFI Input Signals........................................................................................ 7-4

PFI Filters ...................................................................................................................... 7-4

I/O Protection ................................................................................................................ 7-6

Programmable Power-Up States.................................................................................... 7-6

Static DIO on USB-6210/6211/6215/6218 Devices ....................................... 6-2

I/O Protection on USB-6210/6211/6215/6218 Devices.................................. 6-2

Increasing Current Drive on USB-6210/6211/6215/6218 Devices ................ 6-3

Connecting Digital I/O Signals on USB-6210/6211/6215/6218 Devices....... 6-3

Getting Started with DIO Applications in Software on

USB-6210/6211/6215/6218 Devices ........................................................... 6-4

Static DIO on USB-6212/6216 Devices ......................................................... 6-5

I/O Protection on USB-6212/6216 Devices....................................................6-5

Programmable Power-Up States on USB-6212/6216 Devices....................... 6-6

Increasing Current Drive on USB-6212/6216 Devices................................... 6-6

Connecting Digital I/O Signals on USB-6212/6216 Devices ......................... 6-6

Getting Started with DIO Applications in Software on

USB-6212/6216 Devices.............................................................................. 6-7

Chapter 8 Counters

Counter Input Applications ........................................................................................... 8-2

Counting Edges ............................................................................................... 8-2

Single Point (On-Demand) Edge Counting ...................................... 8-2

Buffered (Sample Clock) Edge Counting......................................... 8-3

Controlling the Direction of Counting.............................................. 8-4

Pulse-Width Measurement.............................................................................. 8-4

Single Pulse-Width Measurement .................................................... 8-4

Buffered Pulse-Width Measurement ................................................ 8-5

Period Measurement ....................................................................................... 8-6

Single Period Measurement.............................................................. 8-6

Buffered Period Measurement.......................................................... 8-7

NI USB-621x User Manual viii ni.com

Contents

Semi-Period Measurement ..............................................................................8-8

Single Semi-Period Measurement.....................................................8-8

Buffered Semi-Period Measurement.................................................8-8

Frequency Measurement .................................................................................8-9

Choosing a Method for Measuring Frequency .................................8-13

Position Measurement .....................................................................................8-15

Measurements Using Quadrature Encoders ......................................8-15

Measurements Using Two Pulse Encoders .......................................8-17

Two-Signal Edge-Separation Measurement....................................................8-18

Single Two-Signal Edge-Separation Measurement ..........................8-18

Buffered Two-Signal Edge-Separation Measurement ......................8-19

Counter Output Applications .........................................................................................8-20

Simple Pulse Generation .................................................................................8-20

Single Pulse Generation ....................................................................8-20

Single Pulse Generation with Start Trigger ......................................8-20

Retriggerable Single Pulse Generation .............................................8-21

Pulse Train Generation ....................................................................................8-22

Continuous Pulse Train Generation ..................................................8-22

Frequency Generation .....................................................................................8-23

Using the Frequency Generator ........................................................8-23

Frequency Division .........................................................................................8-24

Pulse Generation for ETS................................................................................8-24

Counter Timing Signals .................................................................................................8-25

Counter n Source Signal..................................................................................8-26

uting a Signal to Counter n Source...............................................8-26

Routing Counter n Source to an Output Terminal ............................8-26

Counter n Gate Signal .....................................................................................8-27

Routing a Signal to Counter n Gate ..................................................8-27

Routing Counter n Gate to an Output Terminal................................8-27

Counter n Aux Signal ......................................................................................8-27

Routing a Signal to Counter n Aux ...................................................8-27

Counter n A, Counter n B, and Counter n Z Signals.......................................8-28

Routing Signals to A, B, and Z Counter Inputs ................................8-28

Counter n Up_Down Signal ............................................................................8-28

Counter n HW Arm Signal ..............................................................................8-28

Routing Signals to Counter n HW Arm Input...................................8-29

Cou

nter n Internal Output and Counter n TC Signals .....................................8-29

Routing Counter n Internal Output to an Output Terminal...............8-29

Frequency Output Signal.................................................................................8-29

Routing Frequency Output to a Terminal .........................................8-29

Default Counter/Timer Pinouts......................................................................................8-30

Counter Triggering ........................................................................................................8-31

Contents

Other Counter Features.................................................................................................. 8-32

Sample Clock .................................................................................................. 8-32

Cascading Counters......................................................................................... 8-33

Counter Filters................................................................................................. 8-33

Prescaling ........................................................................................................ 8-34

Duplicate Count Prevention ............................................................................ 8-35

Example Application That Works Correctly

(No Duplicate Counting) ............................................................... 8-35

Example Application That Works Incorrectly

(Duplicate Counting) ..................................................................... 8-36

Example Application That Prevents Duplicate Count...................... 8-36

Enabling Duplicate Count Prevention in NI-DAQmx...................... 8-37

Chapter 9 Isolation and Digital Isolators on USB-6215/6216/6218 Devices

Digital Isolation............................................................................................................. 9-2

Benefits of an Isolated DAQ Device ............................................................................. 9-2

Reducing Common-Mode Noise................................................................................... 9-3

Creating an AC Return Path............................................................................9-3

Isolated Systems ............................................................................... 9-4

Non-Isolated Systems ....................................................................... 9-4

Chapter 10 Digital Routing and Clock Generation

80 MHz Timebase ......................................................................................................... 10-1

20 MHz Timebase ......................................................................................................... 10-1

100 kHz Timebase......................................................................................................... 10-1

Chapter 11 Bus Interface

USB Signal Stream........................................................................................................ 11-1

Data Transfer Methods .................................................................................................. 11-1

Changing Data Transfer Methods ................................................................... 11-2

Chapter 12 Triggering

Triggering with a Digital Source ................................................................................... 12-1

NI USB-621x User Manual x ni.com

Appendix A Device-Specific Information

USB-6210 ......................................................................................................................A-2

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

USB-6212/6216 Screw Terminal...................................................................................A-6

USB-6212/6216 Mass Termination ...............................................................................A-8

USB-6218 ......................................................................................................................A-13

Appendix B Troubleshooting

Appendix C Technical Support and Professional Services

Glossary

Index

Contents

Device Pinouts

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

Figure A-2. USB-6211/6215 Pinout .........................................................................A-4

Figure A-3. USB-6212/6216 Screw Terminal Pinout...............................................A-6

Figure A-4. USB-6212/6216 Mass Termination Pinout ...........................................A-9

Figure A-5. USB-6218 Pinout ..................................................................................A-13

About This Manual

The NI USB-621x User Manual contains information about using the National Instruments USB-621x data acquisition (DAQ) devices with NI-DAQmx 8.7.1 and later. NI USB-6210, USB-6211, USB-6212, USB-6215, USB-6216, and USB-6218 devices feature up to 32 analog input (AI) channels, up to two analog output (AO) channels, two counters, and up to eight lines of digital input (DI) and up to eight lines of digital output (DO), or 32 bidirectional static DIO lines.

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 for 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.

About This Manual

Platform Text in this font denotes a specific platform and indicates that the text

following it applies only to that platform.

Installing NI-DAQmx

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

Installing Other Software

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

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

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 USB-621x device pinouts.

Chapter 1 Getting Started

Device Specifications

Refer to the NI USB-621x Specifications, available on the NI-DAQ Device Documentation Browser or from

ni.com/manuals, for more detailed

information about USB-621x devices.

Applying Signal Labels to the USB-621x

Your USB-621x kit includes labels for the combicon connectors on USB-621x Screw Terminal devices. You can choose labels with pin numbers or signal names, or blank labels. Choose one of the labels, align the correct label with the terminals printed on the top panel of your device or the 16-position combicon connector, and apply the label, as shown in Figure 1-1.

P/N 19XXXX REVX

msi 6000

1 Terminal Number Label 2 Single-Ended Signal Name Label

3 Differential Signal Name Label 4 User-Defined Custom Label

Figure 1-1. USB-621x Signal Labels

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

USB Cable Strain Relief

You can provide strain relief for the USB cable in the following ways:

• Cable Strain Relief Groove Method—Press the USB cable into one

of the two grooves on the underside of the USB-621x. Choose the USB cable groove that matches your USB cable size, as shown in Figure 1-2a.

• Zip Tie Method—Thread a zip tie through the zip tie bar on the

underside of the USB-621x and tighten around the USB cable, as shown in Figure 1-2b.

Chapter 1 Getting Started

1 USB Cable Strain Relief Groove (Large) 2 USB Cable Strain Relief Groove (Small) 3 USB Cable

4 Zip Tie 5 Zip Tie Bar

Figure 1-2. USB Cable Strain Relief Options

Chapter 1 Getting Started

Mounting the USB-621x

You can use the USB-621x on a desktop or mount it to a standard DIN rail or a panel.

Desktop Use

You can use the USB-621x on a desktop. The USB-621x has grooves on the underside that allow it to be stacked with other like-sized USB-621x

devices.

For secure desktop use. adhere the supplied rubber non-skid feet to the underside of the device, as shown in Figure 1-3.

Note Do not apply the rubber feet if you are panel mounting the USB-621x or stacking the

device on another USB-621x device.

Figure 1-3. Applying Rubber Feet to the USB-621x

DIN Rail Mounting

The DIN rail mounting kit (part number 779689-01, not included in your USB-621x kit) is an accessory you can use to mount the USB-621x family of products to a standard DIN rail.

Note Apply strain relief, as described in the USB Cable Strain Relief section, before

mounting the USB-621x to a DIN rail.

NI USB-621x User Manual 1-4 ni.com

Panel Mounting

Chapter 1 Getting Started

To mount the USB-621x to a board or panel, complete the following steps while referring to Figure 1-4.

Figure 1-4. Mounting the USB-621x on a Panel

Note

Do not apply the rubber feet to the USB-621x when panel mounting the device.

Note Apply strain relief, as described in the USB Cable Strain Relief section, before panel

mounting the USB-621x.

1. Download and print the panel mounting template PDF attached in the KnowledgeBase document, USB-621x Panel Mounting Template. Go to

ni.com/info and enter the info code ex3x98 to locate the

KnowledgeBase.

2. Using the template, mark the bottom point and top point on the panel.

(USB-621x Screw Terminal Devices) The points will be 171.45 mm

(6.75 in.) from each other.

(USB-621x Mass Termination Devices) The points will be 182.56 mm

(7.188 in.) from each other.

Chapter 1 Getting Started

3. Remove the USB cable from the connector on the USB-621x.

4. Screw a #8 or M4 screw into the bottom point on the panel.

5. Set the USB-621x on the screw by fitting it into the bottom screw notch on the underside of the USB-621x.

6. Screw a #8 or M4 screw through the USB-621x top screw hole into the panel.

NI USB-621x User Manual 1-6 ni.com

DAQ System Overview

Figure 2-1 shows a typical DAQ system, which includes the USB-621x device, programming software, and PC (DAQ systems involving the USB-621x Mass Termination device can also include signal conditioning devices and a cable for accessory connection). The following sections contain more information about the components of a typical DAQ system.

DAQ

Hardware

Figure 2-1. Components of a Typical DAQ System

DAQ

Software

Personal Computer

or Laptop

DAQ Hardware

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-621x devices.

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, DO, and DIO signals

• USB Hi-Speed 2.0 interface

•Up to four USB Signal Streams for acquisition and generation

Digital

Routing

and Clock

Generation

functions

Isolation

Barrier

(USB-6215/

6216/6218

devices only)

Digital

Isolators

Figure 2-2. USB-621x Block Diagram

Bus

Interface

Bus

Calibration Circuitry

The USB-621x 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 re t urn 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 Help in version 8.0 or later for more information about using calibration constants.

For a detailed calibration procedure for USB-621x devices, refer to the B/E/M/S Series Calibration Procedure for NI-DAQmx by clicking Manual Calibration Procedures on

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.

Chapter 2 DAQ System Overview

ni.com/calibration.

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).

To mea s ure 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.

ni.com/sensors.

Chapter 2 DAQ System Overview

•If you are using other application software, refer to Common Sensors in the NI-DAQmx Help or the LabVIEW Help in version 8.0 or later.

Cables and Accessories

Cable and accessory options for USB-621x devices are as follows:

• Combicon Accessory for USB-621x Screw Terminal Devices (Optional)—Your USB-621x kit includes combicon connectors with screws and signal labels. The NI USB-621x Accessory Kit (part number 779807-01) contains four additional combicon connectors with screws, a screwdriver, and additional signal labels. You can use the combicon accessory to create custom connection solutions for USB-621x Screw Terminal devices.

• Cables and Accessories for USB-621x Mass Termination Devices—Refer to the USB-6212/6216 Mass Termination Cables and

Accessories section of Appendix A, Device-Specific Information, for a

list of cables and accessories for USB-621x Mass Termination devices.

USB-621x Mass Termination Custom Cabling

NI offers cables and accessories for many applications. However, if you want to develop your own cable, adhere to the following guidelines for best results:

• For AI signals, use shielded, twisted-pair wires for each AI pair of differential inputs. Connect the shield for each signal pair to the ground reference at the source.

•Route the analog lines separately from the digital lines.

•When using a cable shield, use separate shields for the analog and digital sections of the cable. Failure to do so results in noise coupling into the analog signals from transient digital signals.

For more information about the connectors used for DAQ devices, refer to the KnowledgeBase document, Specifications and Manufacturers for Board Mating Connectors, by going to code

rdspmb.

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

ni.com/info and entering the info

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.

USB-621x devices use the NI-DAQmx driver. 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. Yo u 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.

Chapter 2 DAQ System Overview

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

zone.ni.com.

Connector and LED Information

The I/O Connector Signal Descriptions and +5 V Power sections contain information about NI USB-621x connectors. The PWR/ACT LED Indicator section contains information about the NI USB-621x PWR/ACT LED. Refer to Appendix A, Device-Specific Information, for device I/O connector pinouts. Refer to the Applying Signal Labels to the USB-621x section of Chapter 1, Getting Started, for information about applying signal labels.

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 ri e 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:

<AI1,AI9>, <AI2,AI10>, <AI3,AI11>, <AI4,AI12>, <AI5,AI13>, <AI6,AI14>, <AI7,AI15>, <AI16,AI24>, <AI 17, AI 25>, <AI 18, AI 26>, <AI 19, AI 27>, <AI 20, AI 28>, <AI 21, AI 29>, <AI 22, AI 30>, <AI 23, AI 31>

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

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

Chapter 3 Connector and LED Information

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

Signal Name Reference Direction Description

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

voltage output of AO channels 0 to 1.

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>

D GND Input (USB-6210/6211/6215/6218 Devices) Programmable

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

Function Interface or Static Digital Input Channels 0 to 7—Each PFI terminal can be used to supply an

external source for AI, AO, or counter/timer inputs.

Yo u also can use these terminals as static digital input lines.

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

P0.<0..15> D GND Input or

PFI <0..7>/P1.<0..7>, PFI <8..15>/P2.<0..7>

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

D GND Output (USB-6210/6211/6215/6218 Devices) Programmable

Output

D GND Input or

Output

Function Interface or Static Digital Output Channels 0 to 7—You can route many different internal AI,

AO, or counter/timer outputs to each PFI terminal.

Yo u also can use these terminals as static digital output lines.

(USB-6212/6216 Devices) Port 0 Digital I/O Channels 0 to 15—You can individually configure each signal

as an input or output.

(USB-6212/6216 Devices) Programmable Function Interface or Digital I/O Channels 0 to 15—Each of these terminals can

be individually configured as a PFI terminal or a digital I/O terminal.

As a PFI input, each terminal can be used to supply an external source for AI, AO, DI, and DO timing signals or counter/timer inputs.

As a PFI output, you can route many different internal AI, AO, DI, or DO timing signals to each PFI terminal. You also can route the counter/timer outputs to each PFI terminal.

As a Port 1 or Port 2 digital I/O signal, you can individually configure each signal as an input or output.

Chapter 3 Connector and LED Information

+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. The USB-621x monitors the total current and drops the voltage on all of the digital outputs and the +5 V terminals if the 50 mA limit is exceeded.

+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 fuse to protect them from short circuit conditions terminal, you can connect the external power supply to one terminal and use the other as a power source.

. If your USB-621x device has more than one +5 V

USB Device Fuse Replacement

(USB-621x Mass Termination Devices) USB-621x Mass Termination devices

have a replaceable 0.75A,125V fuse (Littelfuse part number 0453.750) that protects the device from overcurrent through the +5 V terminal(s).

To replace a broken fuse in the USB-621x Mass Termination, complete the following steps while referring to Figure 3-1.

1. Remove the USB cable and any I/O signal wires from the device.

2. Remove the four Phillips screws on the bottom of the device to remove the device top. You may have to remove the rubber feet.

USB-621x Screw Terminal devices have a 350 mA self-resetting fuse. USB-621x Mass Termination devices have a 750 mA user-replaceable socketed fuse.

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

Chapter 3 Connector and LED Information

3. Replace the broken fuse in the socket. Figure 3-1 shows the fuse location.

S/s

ction I/O

ltif

it, 400 k

6216

16-

NI USB-

Isolatoed M

16 Inp

INSTRUMENTS

NATIONAL

1 0.75A,125V Fuse, Socketed

Figure 3-1. USB-621x Mass Termination Fuse Location

4. Replace the device top and reattach with the screws.

Note Unscrewing and reinstalling the thread-forming screws over time will produce a

compromised connection between the device top and bottom.

Chapter 3 Connector and LED Information

PWR/ACT LED Indicator

The PWR/ACT LED indicator indicates device status. Table 3-2 shows the behavior of the PWR/ACT LED.

Table 3-2. PWR/ACT LED Status

LED State Device Status

Not lit Device not powered or device error. Refer to ni.com/support if

device is powered.

On, not blinking Device error. Refer to ni.com/support.

Single-blink Operating normally. Connected to USB Hi-Speed port. Refer to the

NI USB-621x Specifications for more information.

Double-blink Connected to USB Full-Speed port. Device performance might be

affected. Refer to the NI USB-621x Specifications for more information.

NI USB-621x User Manual 3-5 ni.com

Analog Input

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

AI <0..n>

AI SENSE

I/O Connector

Mux

DIFF, RSE,

or NRSE

AI GND

AI Ground-Reference

Settings

NI-PGIA

Input Range

Selection

Isolation

Barrier

(USB-6215/

6216/6218

devices only)

ADC

Figure 4-1. USB-621x Analog Input Circuitry

AI FIFO

Digital

Isolators

AI Data

The main blocks featured in the USB-621x analog input circuitry are as follows:

• I/O Connector—You can connect analog input signals to the USB-621x 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 USB-621x 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.

• AI Ground-Reference Settings—The analog input ground-reference settings circuitry selects between differential (DIFF), referenced single-ended (RSE), and non-referenced single-ended (NRSE) input modes. Each AI channel can use a different mode.

• NI-PGIA—The NI programmable gain instrumentation amplifier (NI-PGIA) is a measurement and instrument class amplifier that

Chapter 4 Analog Input

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.

USB-621x devices use the NI-PGIA to deliver high accuracy even when sampling multiple channels with small input ranges at fast rates. USB-621x 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.

• ADC—The analog-to-digital converter (ADC) digitizes the AI signal

by converting the analog voltage into a digital number.

• AI FIFO—USB-621x 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. USB-621x devices can handle multiple A/D conversion operations with DMA, interrupts, or programmed I/O.

• Isolation Barrier and Digital Isolators—Refer to Chapter 9,

Isolation and Digital Isolators on USB-6215/6216/6218 Devices,

for more information.

Analog Input Range

The input range affects the resolution of the USB-621x device for an AI channel. For example, a 16-bit ADC converts analog inputs into one of 65,536 (= 2

) codes—that is, one of 65,536 possible digital values. So, for an input range of –10 V to 10 V, the voltage of each code of a 16-bit ADC is:

10 V 10 V–()–()

------------------------------------------- 305 μV=

USB-621x devices use a calibration method that requires some codes (typically about 5% of the codes) to lie outside of the specified range. This 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.

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

For more information about setting ranges, refer to the NI-DAQmx Help or the LabVIEW Help in version 8.0 or later.

The following table shows the input ranges and resolutions supported by USB-621x devices.

Nominal Resolution Assuming

Input Range

–10 V to 10 V 320 μV

–5 V to 5 V 160 μV

–1 V to 1 V 32 μV

–200 mV to 200 mV 6.4 μV

Analog Input Ground-Reference Settings

USB-621x devices support the following analog input ground-reference settings:

• Differential mode—In DIFF mode, USB-621x devices measure the

difference in voltage between two AI signals.

• Referenced single-ended mode—In RSE mode, USB-621x devices

measure the voltage of an AI signal relative to AI GND.

• Non-referenced single-ended mode—In NRSE mode, USB-621x

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

Chapter 4 Analog Input

5% Over Range

The AI ground-reference setting determines how you should connect your AI signals to the USB-621x device. For more information, refer to one of the following sections depending on your device:

• Connecting Analog Input Signals on USB-6210/6211/6212 Devices

• Connecting Analog Input Signals on USB-6215/6216/6218 Devices

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.

USB-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

Chapter 4 Analog Input

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>

Measured

Voltage

–

PGIA

in–

= [V

– V–] × Gain

Figure 4-2. NI-PGIA

Table 4-1 shows how signals are routed to the NI-PGIA.

Table 4-1. 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 pairs that form differential input channels, refer to the I/O Connector Signal

Descriptions section of Chapter 3, Connector and LED Information.

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

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 NI USB-621x Specifications. 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.

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

Configuring AI Ground-Reference Settings in Software

You can program channels on an USB-621x device to acquire with different ground references.

Chapter 4 Analog Input

To en a b le multimode scanning in LabVIEW, use

Virtual Channel.vi

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 con f i g ure 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 co n f i g ure 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.

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

Multichannel Scanning Considerations

NI-DAQmx Create

USB-621x 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 USB-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 NI USB-621x Specifications lists settling time.

USB-621x devices are designed to have fast settling times. However, several factors can increase the settling time which decreases the accuracy

Chapter 4 Analog Input

of your measurements. To ensure fast settling times, you should do the following (in order of importance):

• 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.

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 and entering the info code

rdbbis.

ni.com/info

• Use Short High-Quality Cabling—Using short high-quality cables

can minimize several effects that degrade accuracy including crosstalk, transmission line effects, and noise. The capacitance of the cable also can increase the settling time.

National Instruments recommends using individually shielded, twisted-pair wires that are 2 m or less to connect AI signals to the device. Refer to the Connecting Analog Input Signals on

USB-6210/6211/6212 Devices or Connecting Analog Input Signals on USB-6215/6216/6218 Devices section for more information.

• 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

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

Chapter 4 Analog Input

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.

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 –200 mV to 200 mV.

Yo u 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.

Chapter 4 Analog Input

• 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. Consider the following examples:

– 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.

– 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 channel 0 then read 100 points from channel 1. The second method switches between channels much often and is affected less by settling time.

also could read all 100 points from

Analog Input Data Acquisition Methods

When performing analog input measurements, you either can perform software-timed or hardware-timed acquisitions:

• 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.

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Chapter 4 Analog Input

• Hardware-Timed Acquisitions—With hardware-timed acquisitions,

a digital hardware signal, AI Sample Clock, 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 are buffered. In a buffered acquisition, data is moved from the DAQ device’s onboard FIFO memory to a PC buffer using 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. After the specified number of samples has been read in, the acquisition 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 acqu

iring 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.

Chapter 4 Analog Input

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.

A digital trigger can initiate these actions. All USB-621x devices support digital triggering. USB-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.

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

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, USB-621x devices have a flexible timing engine. Figure 4-4 summarizes all of the timing options provided by the analog input timing engine.

20 MHz Timebase

100 kHz Timebase

AI Sample

Clock

Timebase

Analog Comparison Event

Ctr n Internal Output

SW Pulse

Programmable

Clock

Divider

Chapter 4 Analog Input

AI Sample Clock

AI Convert Cloc

AI Convert

Clock

Timebase

Ctr n Internal Output

Programmable

Clock

Divider

Figure 4-4. Analog Input Timing Options

USB-621x devices use AI Sample Clock (ai/SampleClock) and AI Convert Clock (ai/ConvertClock) to perform interval sampling. As Figure 4-5 shows, AI Sample Clock controls the sample period, which is determined by the following equation:

1/Sample Period = Sample Rate

Chapter 4 Analog Input

Note The sampling rate is the fastest you can acquire data on the device and still achieve

accurate results. For example, if a USB-621x 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.

Channel 0

Channel 1

Convert Period

Sample Period

Figure 4-5. Interval Sampling

AI Convert Clock controls the Convert Period, which is determined by the following equation:

1/Convert Period = Convert Rate

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 Sample Clock, until the value reaches zero and all desired samples have been acquired.

AI Start Trigger

AI Sample Clock

AI Convert Clock

Sample Counter

Figure 4-6. Posttriggered Data Acquisition Example

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

1 3 0 4 2

Chapter 4 Analog Input

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 Start Trigger (ai/StartTrigger) can be either a hardware or software signal. If AI Start Trigger is set up to be a software start trigger, an output pulse appears on the AI Start Trigger line when the acquisition begins. When the AI Start Trigger 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 Sample Clock, until the value reaches zero. The sample counter is then loaded with the number of posttriggered samples, in this example, three.

AI Start Trigger

AI Reference Trigger

AI Sample Clock

AI Convert Clock

Scan Counter

Figure 4-7. Pretriggered Data Acquisition Example

n/a

0 1 2

1 0 2 2 2

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

USB-621x 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

Chapter 4 Analog Input

AI Sample Clock Signal

Use the AI Sample Clock (ai/SampleClock) signal to initiate a set of measurements. Your USB-621x device samples the AI signals of every channel in the task once for every AI Sample Clock. A measurement acquisition consists of one or more samples.

Yo u can specify an internal or external source for AI Sample Clock. You also can specify whether the measurement sample begins on the rising edge or falling edge of AI Sample Clock.

Using an Internal Source

One of the following internal signals can drive AI Sample Clock:

•Counter n Internal Output

• AI Sample Clock Timebase (divided down)

•A pulse initiated by host software

A programmable internal counter divides down the sample clock timebase.

Using an External Source

Use any input PFI line as the source of AI Sample Clock.

Routing AI Sample Clock to an Output Terminal

You can route AI Sample Clock out to any output PFI terminal. This pulse is always active high.

Yo u 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 Sample Clock.

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 Sample Clock when a measurement acquisition is not in progress. During a measurement acquisition, you can cause your DAQ device to ignore AI Sample Clock using the AI Pause Trigger signal.

A counter on your device internally generates AI Sample Clock unless you select some external source. AI Start Trigger starts this counter and either

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

Chapter 4 Analog Input

software or hardware can stop it once a finite acquisition completes. When using an internally generated AI Sample Clock, you also can specify a

configurable delay from AI Start Trigger to the first AI Sample Clock pulse. By default, this delay is set to two ticks of the AI Sample Clock Timebase signal. When using an externally generated AI Sample Clock, you must ensure the clock signal is consistent with respect to the timing requirements of AI Convert Clock. Failure to do so may result in AI Sample Clock pulses that are masked off and acquisitions with erratic sampling intervals. Refer to the AI Convert Clock Signal section for more information about the timing requirements between AI Convert Clock and AI Sample Clock.

Figure 4-8 shows the relationship of AI Sample Clock to AI Start Trigger.

AI Sample Clock Timebase

AI Start Trigger

AI Sample Clock

Delay From

Start

Tr i

Figure 4-8. AI Sample Clock and AI Start Trigger

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

•

(USB-6210/6211/6215 Devices) PFI <0..3>

•

(USB-6212/6216 Devices) PFI <0..15>

•

(USB-6218 Devices) PFI <0..3>, PFI <8..11>

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

Chapter 4 Analog Input

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.

Yo u can specify either an internal or external signal as the source of AI Convert Clock. You also can specify whether the measurement sample begins on the rising edge or falling edge of AI Convert Clock.

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 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 Convert Clock:

• AI Convert Clock Timebase (divided down)

•Counter n Internal Output

A programmable internal counter divides down the AI Convert Clock Timebase to generate AI Convert Clock. Started by AI Sample Clock, the counter counts down to zero, produces an AI Convert Clock, reloads itself, and repeats the process until the sample is finished. It then reloads itself in preparation for the next AI Sample Clock pulse.

Using an External Source

Use any input PFI line as the source of AI Convert Clock.

Routing AI Convert Clock to an Output Terminal

You can route AI Convert Clock (as an active low signal) out to any output PFI terminal.

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

AI Convert Clock Timebase

AI Sample Clock

AI Convert Clock

Chapter 4 Analog Input

Using a Delay from Sample Clock to Convert Clock

When using an internally generated AI Convert Clock, you also can specify a configurable delay from AI Sample Clock to the first AI Convert Clock pulse within the sample. By default, this delay is three ticks of AI Convert Clock Timebase.

Figure 4-9 shows the relationship of AI Sample Clock to AI Convert Clock.

Delay From

Sample

Clock

Convert

Perio d

Figure 4-9. AI Sample Clock and AI Convert Clock

Other Timing Requirements

The sample and conversion level timing of USB-621x devices work such that clock signals are gated off unless the proper timing requirements are met. For example, the device ignores both AI Sample Clock and AI Convert Clock until it receives a valid AI Start Trigger signal. Once the device recognizes an AI Sample Clock pulse, it ignores subsequent AI Sample Clock pulses until it receives the correct number of AI Convert Clock pulses.

Similarly, the device ignores all AI Convert Clock pulses until it recognizes an AI Sample Clock pulse. Once the device receives the correct number of AI Convert Clock pulses, it ignores subsequent AI Convert Clock pulses until it receives another AI Sample Clock. 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 Sample Clock and AI Convert Clock.

Chapter 4 Analog Input

AI Sample Clock

AI Convert Clock

Channel Measured 1 2 3 0

Figure 4-10. AI Sample Clock Too Fast For AI Convert Clock;

AI Sample Clock Pulses Are Gated Off

Sample Clock

AI Convert Clock

1 2 3 0 1 2 3 0

Sample #1 Sample #2 Sample #3

AI Sample Clock

AI Convert Clock

Channel Measured

1 2 3 0

Sample #1 Sample #2 Sample #3

1 2 3 0 1 2 3 0

Figure 4-11. AI Convert Clock Too Fast For AI Sample Clock;

AI Convert Clock Pulses Are Gated Off

1 2 3 0

Sample #1 Sample #2 Sample #3

1 2 3 0

Figure 4-12. AI Sample Clock and AI Convert Clock Improperly Matched;

Leads To Aperiodic Sampling

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

AI Sample Clock

AI Convert Clock

Chapter 4 Analog Input

Channel Measured 1 2 3 0

Figure 4-13. AI Sample Clock and AI Convert Clock Properly Matched

A single external signal can drive both AI Sample Clock and AI Convert Clock at the same time. In this mode, each tick of the external clock causes a conversion on the ADC. Figure 4-14 shows this timing relationship.

AI Sample Clock

AI Convert Clock

Channel Measured

Figure 4-14. Single External Signal Driving AI Sample Clock and

AI Convert Clock Timebase Signal

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

• AI Sample Clock Timebase

• 20 MHz Timebase

1 2 3 0 1 2 3 0

Sample #1 Sample #2 Sample #3

1 2 3 0

Sample #1 Sample #2 Sample #3

AI Convert Clock Simultaneously

1 2 3 0 1 … 0

AI Convert Clock Timebase is not available as an output on the I/O connector.

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 Hold Complete Event out to any output PFI terminal.

Chapter 4 Analog Input

The polarity of AI Hold Complete Event 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. After 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.

Using a Digital Source

To use AI Start Trigger with a digital source, specify a source and an edge. The source can be any of the following signals:

•

•Counter n Internal Output

(USB-6210/6211/6215 Devices) PFI <0..3>

(USB-6212/6216 Devices) PFI <0..15>

(USB-6218 Devices) PFI <0..3>, PFI <8..11>

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 Help in version 8.0 or later for more information.

Yo u also can specify whether the measurement acquisition begins on the rising edge or falling edge of AI Start Trigger.

Routing AI Start Trigger to an Output Terminal

You can route AI Start Trigger out to any output PFI terminal.

The output is an active high pulse.

The device also uses AI Start Trigger to initiate pretriggered DAQ operations. In most pretriggered applications, a software trigger generates AI Start Trigger. Refer to the AI Reference Trigger Signal section for a

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

complete description of the use of AI Start Trigger and AI Reference Trigger 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.

After 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.

Chapter 4 Analog Input

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

Chapter 4 Analog Input

Using a Digital Source

To use AI Reference Trigger with a digital source, specify a source and an edge. The source can be any input PFI signal.

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 Help in version 8.0 or later for more information.

Yo u also can specify whether the measurement acquisition stops on the rising edge or falling edge of AI Reference Trigger.

Routing AI Reference Trigger to an Output Terminal

You can route AI Reference Trigger out to any output PFI 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.

Using a Digital Source

To use AI Sample Clock, specify a source and a polarity. The source can be any input PFI signal.

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 Help in version 8.0 or later for more information.

Getting Started with AI Applications in Software

You can use the USB-621x device in the following analog input applications:

• Single-point analog input

• Finite analog input

• Continuous analog input

Yo u 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.

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

Note For more information about programming analog input applications and triggers in

software, refer to the NI-DAQmx Help or the LabVIEW Help in version 8.0 or later.

Connecting Analog Input Signals on USB-6210/6211/6212 Devices

Table 4-2 summarizes the recommended input configuration for both types of signal sources on USB-6210/6211/6212 devices.

Chapter 4 Analog Input

Table 4-2. USB-6210/6211/6212 Analog Input Configuration

Floating Signal Sources

(Not Connected to Building Ground)

Ground-Referenced

Signal Sources

†

AI Ground-Reference

Setting

Differential (DIFF)

Non-Referenced Single-Ended (NRSE)

Referenced Single-Ended (RSE)

Examples:

•Ungrounded thermocouples

• Signal conditioning with isolated outputs

• Battery devices

Signal Source DAQ Device

AI+

+ –

Signal Source DAQ Device

+ –

Signal Source DAQ Device

+ –

AI–

–

AI GND

–

AI SENSE

AI GND

–

AI GND

Example:

•Plug-in instruments with non-isolated outputs

DAQ Device

AI+

AI–

AI GND

AI SENSE

AI GND

DAQ DeviceSignal Source

AI GND

–

Signal Source

+ –

Signal Source DAQ Device

+ –

NOT RECOMMENDED for the

USB-6210/6211/6212

+ –

Ground-loop potential (VA – VB) are added

to measured signal.

Refer to the Analog Input Ground-Reference Settings section for descriptions of the RSE, NRSE, and DIFF modes and

software considerations.

†

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

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

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 3 m (10 ft).

• 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.

Chapter 4 Analog Input

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).

Chapter 4 Analog Input

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

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 4-26 ni.com

Chapter 4 Analog Input

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

Impedance

<100 Ω

–

AI–

AI SENSE

AI GND

Figure 4-16. Differential Connections for Floating Signal Sources

without Bias Resistors

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 4 Analog Input

Floating

Signal

Source

R is about 100 times source impedance of sensor

–

AI+

AI–

AI SENSE

AI GND

Figure 4-17. Differential Connections for Floating Signal Sources

with Single Bias Resistor

Yo u can fully balance the signal path by connecting another resistor of the same value between the positive input and AI GND on the USB-6210/6211/6212 device, as shown in Figure 4-18. 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 4-28 ni.com

Floating

Signal

Source

Bias

Current

Return

Paths

–

Bias Resistors (see text)

Input Multiplexers

AI SENSE

AI+

AI–

Instrumentation

Amplifier

PGIA

–

Chapter 4 Analog Input

Measured

Voltage

–

AI GND

I/O Connector

USB-6210/6211/6212 Device Configured in DIFF Mode

Figure 4-18. 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 inputs; be aware that there is some gain error from loading down the source, as shown in Figure 4-19.

Chapter 4 Analog Input

AC-Coupled

Floating

Signal

Source

AC Coupling

–

AI+

AI–

AI SENSE

AI GND

Figure 4-19. 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 4-20 shows a floating source connected to the DAQ device in NRSE mode.

Floating

Signal

Source

–

SENSE

AI GND

Figure 4-20. 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 4-16, 4-17, 4-18, and 4-19 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 connection is shared with all channels rather than being cabled in a twisted pair with the AI+ signal.

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Chapter 4 Analog Input

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 for more information about the DAQ Assistant.

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

Figure 4-21 shows how to connect a floating signal source to the USB-6210/6211/6212 device configured for RSE mode.

AI <0..n>

Programmable Gain

Floating

Signal

Source

–

I/O Connector

Input Multiplexers

AI SENSE

AI GND

Selected Channel in RSE Configuration

Instrumentation

PGIA

–

Amplifier

Measured

Vol tage

–

Figure 4-21. 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 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.

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

Chapter 4 Analog Input

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 3 m (10 ft).

• 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.

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Chapter 4 Analog Input

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 4-2, there can be a potential difference between AI GND and the ground of the sensor. In RSE mode, this ground loop causes measurement errors.

Using Differential Connections for Ground-Referenced Signal Sources

Figure 4-22 shows how to connect a ground-referenced signal source to the USB-6210/6211/6212 device configured in DIFF mode.

Chapter 4 Analog Input

Ground-

Referenced

Signal

Source

Common-

Mode

Noise and

Ground

Potential

I/O Connector

–

AI–

–

Input Multiplexers

AI SENSE

AI GND

USB-6210/6211/6212 Configured in DIFF Mode

Instrumentation

Amplifier

PGIA

–

Measured

Vol tage

–

Figure 4-22. 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.

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

Figure 4-23 shows how to connect ground-reference signal sources to the USB-6210/6211/6212 device in NRSE mode.

NI USB-621x User Manual 4-34 ni.com

Chapter 4 Analog Input

I/O Connector

Ground-

Referenced

Signal

Source

Common-

Mode Noise

and Ground

Potential

AI <0..15>

or AI <16..n>

–

Input Multiplexers

AI GND

–

USB-6210/6211/6212 Configured in NRSE Mode

AI SENSE

Instrumentation

Amplifier

PGIA

–

Measured

Voltage

–

Figure 4-23. 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 for more information about the DAQ Assistant.

Chapter 4 Analog Input

Connecting Analog Input Signals on USB-6215/6216/6218 Devices

You can connect the USB-6215/6216/6218 directly to a variety of devices and other signal sources. Make sure the devices you connect to the USB-6215/6216/6218 are compatible with the input specifications of the module.

When connecting various sources to the USB-6215/6216/6218, you can use differential, single-ended, or a combination of single-ended and differential connections.

Note Yo u 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.

Taking 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. The AI <0..31> description in Table 3-1, I/O Connector Signals, lists the signal pairs that are valid for differential connection configurations with USB-621x devices.

Figure 4-24 shows a differential connection configuration.

AI 0+

* This signal name indicates the differential pair. Refer to Table 3-1, I/O Connector

Signals, for a list of differential signal pairs.

Figure 4-24. Connecting to the USB-6215/6216/6218 in Differential Mode

NI USB-621x User Manual 4-36 ni.com

AI 0– (AI 8)*

AI 1+

AI 1– (AI 9)*

AI GND

PGIA

Mux

USB-6215/6216/6218

ADC

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

, to be rejected during the measurement of V1.

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

Taking Referenced Single-Ended (RSE) Measurements

Using the RSE measurement configuration allows the USB-6215/6216/6218 to take measurements on all AI channels when all channels share a common ground. Figure 4-25 shows an RSE connection configuration.

Note If you leave the AI GND pin unconnected, the signals float outside the working input

range of the USB-6215/6216/6218. This can result in unreliable measurements because you cannot ensure that the input signal is within 10 V of AI GND.

AI 1

AI 2

Mux

Chapter 4 Analog Input

ADCPGIA

AI GND

USB-6215/6216/6218

Figure 4-25. Connecting to the USB-6215/6216/6218 in RSE Mode

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

Chapter 4 Analog Input

Taking 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 instrumentation amplifier (PGIA) that is shared among all channels configured for NRSE mode. The behavior of this configuration is similar to that of RSE connections, except it provides improved noise rejection. Figure 4-26 shows an NRSE connection configuration.

AI 1

AI 0

AI SENSE

AI GND

Figure 4-26. Connecting to the USB-6215/6216/6218 in NRSE Mode

Mux

PGIA

USB-6215/6216/6218

ADC

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

NI USB-621x User Manual 4-38 ni.com

Analog Output

Most USB-621x devices have analog output functionality. USB-621x devices that support analog output have two AO channels controlled by a single clock and capable of waveform generation. Refer to the NI USB-621x Specifications for information about your device capabilities.

Figure 5-1 shows the analog output circuitry of USB-621x devices.

AO 0

DAC0

AO FIFO

Isolation

Barrier

(USB-6215/

6216/6218

devices only)

Digital

Isolators

AO Data

AO 1

The main blocks featured in the USB-621x analog output circuitry are as follows:

• DAC0 and DAC1—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 DACs. It allows you to download the points of a waveform to your USB-621x device without host computer interaction.

DAC1

AO Sample Clock

Figure 5-1. USB-621x Analog Output Circuitry

Chapter 5 Analog Output

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

AO Sample Clock Signal section for more information.

• Isolation Barrier and Digital Isolators—Refer to Chapter 9,

Isolation and Digital Isolators on USB-6215/6216/6218 Devices,

for more information.

AO Range

The AO range is ±10 V for USB-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 can perform software-timed or hardware-timed generations:

• Software-timed generations—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—A digital hardware signal controls the rate of the generation. This signal can be generated internally on your device or provided externally.

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.

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

Chapter 5 Analog Output

– Hardware-timed acquisitions can use hardware triggering.

Hardware-timed operations are buffered. During hardware-timed AO generation, data is moved from a PC buffer to the onboard FIFO on the USB-621x 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 is not repeated. New data must be continually written to the buffer. If the program does not write new data to the buffer at a fast enough rate to keep up with the generation, the buffer underflows and causes an error.

Analog Output Digital Triggering

Analog output supports two different triggering actions:

• Start trigger

•Pause trigger

Chapter 5 Analog Output

A digital trigger can initiate these actions on 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 5-2 shows how to make AO connections to the device.

Analog Output Channels

Load

V OUT

–

AO 0

AO GND

AO 1

Channel 0

Channel 1

Isolation

Barrier

(USB-6215/

6216/6218

devices only)

Digital

Isolators

USB-621x Device

Figure 5-2. Analog Output Connections

Analog Output Timing Signals

Figure 5-3 summarizes all of the timing options provided by the analog output timing engine.

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

PFI

Chapter 5 Analog Output

PFI

20 MHz Timebase

100 kHz Timebase

USB-621x 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 Sample Clock

Timebase

Figure 5-3. Analog Output Timing Options

Ctr n Internal Output

Programmable

Clock

Divider

Sample Clock

Timebase Divisor

AO Sample Cloc

Using a Digital Source

To use AO Start Trigger, specify a source and an edge. The source can be one of the following signals:

•A pulse initiated by host software

•

(USB-6211/6215 Devices) PFI <0..3>

•

(USB-6212/6216 Devices) PFI <0..15>

•

(USB-6218 Devices) PFI <0..3>, PFI <8..11>

• AI Start Trigger (ai/StartTrigger)

Chapter 5 Analog Output

The source also can be one of several internal signals on your USB-621x device. Refer to Device Routing in MAX in the NI-DAQmx Help or the LabVIEW Help in version 8.0 or later for more information.

Yo u also can specify whether the waveform generation begins on the rising edge or falling edge of AO Start Trigger.

Routing AO Start Trigger to an Output Terminal

You can route AO Start Trigger out to any output PFI terminal. The output is an active high pulse.

AO Pause Trigger Signal

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

If the AO Sample Clock is derived from AO Sample Clock Timebase—for example, when you choose the onboard 20 MHz or 100 kHz Timebase—the AO Sample Clock Timebase is divided down by a programmable clock divider circuit and then drives AO Sample Clock, as shown in Figure 5-3.

In this case, AO Pause Trigger masks off AO Sample Clock Timebase pulses from the programmable clock divider.

For example, an internal timebase is routed to AO Sample Clock Timebase and the Timebase divisor is 5, as shown in Figure 5-4. AO Sample Clock normally asserts once for every five periods of AO Sample Clock Timebase; the programmable clock divider counts down from 4 to 0. When AO Pause Trigger is asserted, the programmable clock divider ignores pulses of AO Sample Clock Timebase.

AO Sample Clock

AO Sample Clock Timebase

Programmable Clock Divider Count

AO Pause Trigger

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

043 21043

The Programmable Clock Divider ignores AO Sample Clock Timebase when AO Pause Trigger is true.

Figure 5-4. AO Pause Trigger Example

210

Chapter 5 Analog Output

If you are using any signal as the source of your sample clock, the generation resumes as soon as AO Pause Trigger is deasserted and another edge of the sample clock is received, as shown in Figure 5-5.

Pause Trigger

Sample Clock

Figure 5-5. AO Pause Trigger with Other Signal Source

Using a Digital Source

To use AO Pause Trigger, specify a source and a polarity. The source can be any input PFI signal.

The source also can be one of several other internal signals on your USB-621x device. Refer to Device Routing in MAX in the NI-DAQmx Help or the LabVIEW Help in version 8.0 or later for more information.

Yo u also can specify whether the samples are paused when AO Pause Trigger 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 Sample Clock. You also can specify whether the DAC update begins on the rising edge or falling edge of AO Sample Clock.

Using an Internal Source

One of the following internal signals can drive AO Sample Clock:

• AO Sample Clock Timebase (divided down)

•Counter n Internal Output

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

Chapter 5 Analog Output

Using an External Source

Use any input PFI line as the source of AO Sample Clock.

Routing AO Sample Clock to an Output Terminal

You can route AO Sample Clock (as an active low signal) out to any output PFI terminal.

Other Timing Requirements

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

Figure 5-6 shows the relationship of AO Sample Clock to AO Start Trigger.

AO Sample Clock Timebase

AO Start Trigger

AO Sample Clock

Delay

From Start

Trigger

Figure 5-6. AO Sample Clock and AO Start Trigger

AO Sample Clock Timebase Signal

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

Yo u can route any of the following signals to be the AO Sample Clock Timebase signal:

•20MHzTimebase

• 100 kHz Timebase

•

(USB-6211/6215 Devices) PFI <0..3>

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

Chapter 5 Analog Output

• (USB-6212/6216 Devices) PFI <0..15>

•

(USB-6218 Devices) PFI <0..3>, PFI <8..11>

AO Sample Clock Timebase is not available as an output on the I/O connector.

Yo u might use AO Sample Clock Timebase 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 Sample Clock rather than AO Sample Clock Timebase.

Getting Started with AO Applications in Software

You can use a USB-621x device in the following analog output applications:

• Single-point (on-demand) generation

• Finite generation

• Continuous generation

• Waveform generation

Yo u 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 Help in version 8.0 or later.

Digital I/O

Refer to one of the following sections, depending on your device:

• Digital I/O on USB-6210/6211/6215/6218 Devices— USB-6210/6211/6215/6218 devices have up to eight lines of digital inputs (DI) and up to eight lines of digital output (DO).

• Digital I/O on USB-6212/6216 Devices—USB-6212/6216 devices

have up to 32 bidirectional static digital I/O (DIO) lines.

Digital I/O on USB-6210/6211/6215/6218 Devices

USB-6210/6211/6215/6218 devices have up to eight static digital input lines, P0.<0..7>. These lines also can be used as PFI inputs.

USB-6210/6211/6215/6218 devices have up to eight static digital output lines, P1.<0..7>. 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 USB-621x Specifications. Refer to Chapter 7,

PFI, for more information on PFI inputs and outputs.

Figure 6-1 shows the circuitry of one DI line and one DO line. The following sections provide information about the various parts of the DIO circuit.

Chapter 6 Digital I/O

Static DI

Static DO

Figure 6-1. USB-6210/6211/6215/6218 Digital I/O Circuitry

I/O Protection

47 kΩ Pull-Down

I/O Protection

47 kΩ Pull-Down

The DI terminals are named P0.<0..7> on the USB-6210/6211/6215/6218 device I/O connector. The DO terminals are named P1.<0..7> on the USB-6210/6211/6215/6218 device I/O connector.

The voltage input and output levels and the current drive levels of the DIO lines are listed in the NI USB-621x Specifications.

Static DIO on USB-6210/6211/6215/6218 Devices

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.

I/O Protection on USB-6210/6211/6215/6218 Devices

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.

P0.

P1.

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

Chapter 6 Digital I/O

Increasing Current Drive on USB-6210/6211/6215/6218 Devices

The total internal current limit for digital outputs and power drawn 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 and LED Information.

Connecting Digital I/O Signals on USB-6210/6211/6215/6218 Devices

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 6-2.

LED

+5 V

When using a

USB-6215/6218,

you must connect

D GND and/or AI GND

to the local ground

on your system.

Caution

Exceeding the maximum input voltage ratings, which are listed in the NI USB-621x Specifications, can damage the DAQ device and the computer. NI is not liable for any damage resulting from such signal connections.

+5 V

Switch

and USB-6218

P1.<0..3>

TTL Signal

D GND

I/O Connector

USB-6210/6211/6215/6218 Device

P0.<0..3>

Figure 6-2. USB-6210/6211/6215/6218 Digital I/O Connections

Isolation

Barrier

(USB-6215

devices only)

Digital

Isolators

Chapter 6 Digital I/O

Getting Started with DIO Applications in Software on USB-6210/6211/6215/6218 Devices

You can use the USB-6210/6211/6215/6218 device in the following digital I/O applications:

• Static digital input

• Static digital output

Note For more information about programming digital I/O applications and triggers in

software, refer to the NI-DAQmx Help or the LabVIEW Help in version 8.0 or later.

Digital I/O on USB-6212/6216 Devices

USB-6212/6216 devices contain:

• Up to 16 DIO signals (P0.<0..15>). Each signal can be individually configured as either:

– Static digital input

– Static digital output

• 16 PFI/DIO signals (PFI <0..7>/P1.<0..7> and PFI <8..15>/P2.<0..7>). Each signal can be individually configured as either:

– Static digital input

– Static digital output

–PFI input

–PFI output

Each pin is called PFI x when used as a PFI; each pin is named P1.x or P2.x when used as a digital input or output.

Figure 6-3 shows the circuitry of one DIO line. Each DIO line is similar. The voltage input and output levels and the current drive levels of the DIO lines are listed in the NI USB-621x Specifications.

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

Static DO

Buffer

Chapter 6 Digital I/O

Digital Line Direction Control

Static DI

Figure 6-3. USB-6212/6216 Digital I/O Circuitry

The following sections provide information about the various parts of the DIO circuit.

Static DIO on USB-6212/6216 Devices

Each of the USB-6212/6216 DIO lines can be used as a static DI or DO line. You can use static DIO lines to monitor or control digital signals. Each DIO can be individually configured as a digital input (DI) or digital output (DO).

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

I/O Protection on USB-6212/6216 Devices

Each DIO 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:

•If you configure a PFI or DIO line as an output, do not connect it to any external signal source, ground signal, or power supply.

•If you configure a PFI or DIO line as an output, understand the current requirements of the load connected to these 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.

•If you configure a PFI or DIO line as an input, do not drive the line with voltages outside of its normal operating range. The PFI or DIO lines have a smaller operating range than the AI signals.

• Treat the DAQ device as you would treat any static sensitive device. Always properly ground yourself and the equipment when handling the DAQ device or connecting to it.

I/O Protection

Weak Pull-Down

P0.x

Chapter 6 Digital I/O

Programmable Power-Up States on USB-6212/6216 Devices

At system startup and reset, the hardware sets all PFI and DIO lines to high-impedance inputs by default. The DAQ device does not drive the signal high or low. Each line has a weak pull-down resistor connected to it, as described in the NI USB-621x Specifications.

NI-DAQmx supports programmable power-up states for PFI and DIO lines. Software can program any value at power up to the P0, P1, or P2 lines. The PFI and DIO lines can be set as:

• A high-impedance input with a weak pull-down resistor (default)

• An output driving a 0

• An output driving a 1

Refer to the NI-DAQmx Help or the LabVIEW Help in version 8.0 or later for more information about setting power-up states in NI-DAQmx or MAX.

Increasing Current Drive on USB-6212/6216 Devices

Connecting Digital I/O Signals on USB-6212/6216 Devices

The DIO signals, P0.<0..15>, P1.<0..7>, and P2.<0..7> are referenced to D GND. You can individually program each line as an input or output. Figure 6-4 shows P0.<0..3> configured for digital input and P1.<0..3> configured for digital output. 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 the figure.

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

LED

Chapter 6 Digital I/O

+5 V

Isolation

Barrier

(USB-6216

devices only)

+5 V

When using a

USB-6216,

you must connect

D GND and/or AI GND

to the local ground

on your system.

Switch

TTL Signal

I/O Connector

P1.<0..3>

P0.<0..3>

D GND

USB-6212/6216 Device

Digital

Isolators

Figure 6-4. USB-6212/6216 Digital I/O Connections

Caution

Getting Started with DIO Applications in Software on USB-6212/6216 Devices

You can use the USB-6212/6216 device in the following digital I/O applications:

• Static digital input

• Static digital output

Note For more information about programming digital I/O applications and triggers in

software, refer to the NI-DAQmx Help or the LabVIEW Help in version 8.0 or later.

PFI

USB-621x devices have multiple Programmable Function Interface (PFI) signals.

Each input PFI can be individually configured as the following:

• A static digital input

• A timing input signal for AI, AO, or counter/timer functions

Each output PFI can be individually configured as the following:

• A static digital output

• A timing output signal from AI, AO, or counter/timer functions

Note (USB-6212/6216 Devices) PFI signals on USB-6212/6216 devices can be configured

as input or output.

Each PFI input also has a programmable debouncing filter. Figure 7-1 shows the circuitry of an input PFI line.

Isolation

Static DI

PFI/DIO Pin

I/O Protection

Weak Pull-Down

PFI

Filters

Figure 7-1. USB-621x PFI Input Circuitry

Barrier

(USB-6215/

6216/6218

devices only)

Digital

Isolators

To Input Timing Signal Selectors

Chapter 7 PFI

Timing Signals

Figure 7-2 shows the circuitry of an output PFI line.

Isolation

Barrier

(USB-6215/

6216/6218

devices only)

Digital

Isolators

Static DO

Buffer

Direction Control

Figure 7-2. USB-621x PFI Output Circuitry

I/O Protection

47 kΩ Pull-Down

When a terminal is used as a timing input or output signal, it is called PFI x. When a terminal is used as a static digital input or output, it is called P0.x, P1.x, or P2.x.

The voltage input and output levels and the current drive levels of the PFI signals are listed in the NI USB-621x Specifications.

Using PFI Terminals as Timing Input Signals

Use PFI terminals to route external timing signals to many different USB-621x functions. Each input PFI terminal can be routed to any of the following signals:

• AI Convert Clock (ai/ConvertClock)

• AI Sample Clock (ai/SampleClock)

• AI Start Trigger (ai/StartTrigger)

• AI Reference Trigger (ai/ReferenceTrigger)

•AI Pause Trigger (ai/PauseTrigger)

• AI Sample Clock Timebase (ai/SampleClockTimebase)

• AO Start Trigger (ao/StartTrigger)

• AO Sample Clock (ao/SampleClock)

PFI/DIO Pin

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

Chapter 7 PFI

• AO Sample Clock Timebase (ao/SampleClockTimebase)

•AO Pause Trigger (ao/PauseTrigger)

•Counter input signals for either counter—Source, Gate, Aux, HW_Arm, A, B, Z

Most functions allow you to configure the polarity of PFI inputs and whether the input is edge or level sensitive.

Exporting Timing Output Signals Using PFI Terminals

You can route any of the following timing signals to any PFI output terminal:

• AI Convert Clock

• AI Hold Complete Event (ai/HoldCompleteEvent)

• AI Reference Trigger (ai/ReferenceTrigger)

• AI Sample Clock (ai/SampleClock)

• AI Start Trigger (ai/StartTrigger)

• AO Sample Clock

• AO Start Trigger (ao/StartTrigger)

•Counter n Source

•Counter n Gate

•Counter n Internal Output

•Frequency Output

(ai/ConvertClock)

(ao/SampleClock)

Note Signals with a * are inverted before being driven to a terminal; that is, these signals

are active low.

Using PFI Terminals as Static Digital I/Os

Each input PFI line can be individually configured as a static digital input, called P0.x. Each output PFI line can be individually configured as a static digital output, called P1.x.

On USB-6212/6216 devices, all PFI lines can be individually configured as static digital inputs or static digital outputs, called

P0.x.

Chapter 7 PFI

Connecting PFI Input Signals

All PFI input connections are referenced to D GND. Figure 7-3 shows this reference, and how to connect an external PFI 0 source and an external PFI 2 source to two PFI terminals.

PFI 0

PFI 2

PFI Filters

Note NI-DAQmx only supports filters on counter inputs.

PFI 0

Source

PFI 2

Source

D GND

I/O Connector

USB-621x Device

Figure 7-3. PFI Input Signals Connections

You can enable a programmable debouncing filter on each PFI signal. When the filters are enabled, your device samples the input on each rising edge of a filter clock. USB-621x devices use an onboard oscillator to generate the filter clock with a 40 MHz frequency.

The following is an example of low to high transitions of the input signal. High to low transitions work similarly.

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Chapter 7 PFI

Assume that an input terminal has been low for a long time. The input terminal then changes from low to high, but glitches several times. When the filter clock has sampled the signal high on N consecutive edges, the low to high transition is propagated to the rest of the circuit. The value of N depends on the filter setting; refer to Table 7-1.

Table 7-1. Filters

Filter Setting

N (Filter Clocks

Needed to

Pass Signal)

Pulse Width

Guaranteed to

Pass Filter

Guaranteed to

Not Pass Filter

125 ns 5 125 ns 100 ns

6.425 μs 257 6.425 μs 6.400 μs

2.56 ms ~101,800 2.56 ms 2.54 ms

Disabled — — —

The filter setting for each input can be configured independently. On power up, the filters are disabled. Figure 7-4 shows an example of a low to high transition on an input that has its filter set to 125 ns (N = 5).

PFI Terminal

Filter Clock

(40 MHz)

Filtered Input

1 2 3 1 4 1 2 3 4 5

Figure 7-4. Filter Example

Filtered input goes high when terminal is sampled high on five consecutive filter clocks.

Pulse Width

Enabling filters introduces jitter on the input signal. For the 125 ns and

6.425 μs filter settings, the jitter is up to 25 ns. On the 2.56 ms setting,

the jitter is up to 10.025 μs.

Refer to the KnowledgeBase document, Digital Filtering with M Series, for more information about digital filters and counters. To access this KnowledgeBase, go to

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

Chapter 7 PFI

I/O Protection

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

•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.

• Treat the DAQ device as you would treat any static sensitive device. Always properly ground yourself and the equipment when handling the DAQ device or connecting to it.

Programmable Power-Up States

At system startup and reset, the hardware sets all output PFI and DO lines to high-impedance by default. The DAQ device does not drive the signal high or low. Each line has a weak pull-down resistor connected to it, as described in the NI USB-621x Specifications.

NI-DAQmx supports programmable power-up states for PFI and DIO lines. Software can program any value at power up to the P1 lines. The output PFI and DO lines can be set as:

• A high-impedance input with a weak pull-down resistor (default)

• An output driving a 0

• An output driving a 1

Refer to the NI-DAQmx Help or the LabVIEW Help in version 8.0 or later for more information about setting power-up states in NI-DAQmx or MAX.

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

Counters

USB-621x devices have two general-purpose 32-bit counter/timers and one frequency generator, as shown in Figure 8-1. 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 8-1. USB-621x Counters

Chapter 8 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 how the software can read the counter contents at any time without disturbing the counting process. Figure 8-2 shows an example of single point edge counting.

Counter Armed

SOURCE

Counter Value 1 0 5 4 3 2

Figure 8-2. Single Point (On-Demand) Edge Counting

Yo u 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.

Yo u 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 8-3 shows an example of on-demand edge counting with a pause trigger.

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

Pause Trigger

(Pause When Low)

SOURCE

Chapter 8 Counters

Counter Armed

Counter Value

1 0 0 5 4 3 2

Figure 8-3. 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.

Yo u 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 8-4 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

1 0 7 6 3 4 5 2

Figure 8-4. Buffered (Sample Clock) Edge Counting

Chapter 8 Counters

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.

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.

Yo u 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.

Yo u 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 is 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 waits 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.

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

Chapter 8 Counters

Figure 8-5 shows an example of a single pulse-width measurement.

GATE

SOURCE

1 0

Counter Value

GATE

SOURCE

Counter Value

Buffer

HW Save Register

Figure 8-5. 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.

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 8-6 shows an example of a buffered pulse-width measurement.

1 0 3

3 3

2 1 2

Figure 8-6. Buffered Pulse-Width Measurement

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

Chapter 8 Counters

condition is not met, consider using duplicate cou nt prevention, described in the Duplicate Count Prevention section.

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.

Yo u 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.

Yo u 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.

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 8-7 shows an example of a single period measurement.

GATE

SOURCE

1 0 3 5 4

Counter Value

HW Save Register

Figure 8-7. Single Period Measurement

NI USB-621x User Manual 8-6 ni.com

GATE

Chapter 8 Counters

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 8-8 shows an example of a buffered period measurement. In this example, a period is defined by two consecutive rising edges.

Counter Armed

SOURCE

Counter Value

Buffer

Time N

t0At 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-621x device stores the counter value in

the buffer.

The rising edge of Gate indicates the end of the second period. The USB-621x device stores the counter value

in the buffer.

1 3

311

3 3

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

Compliance

Contents

About This Manual

Conventions

Related Documentation

NI-DAQmx for Windows

NI-DAQmx Base (Linux/Mac OS X/LabVIEW PDA 8.x)

LabVIEW

LabWindows/CVI

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

Installing NI-DAQmx

Installing Other Software

Installing the Hardware

Device Pinouts

Device Specifications

Applying Signal Labels to the USB-621x

Figure 1-1. USB-621x Signal Labels

USB Cable Strain Relief

Figure 1-2. USB Cable Strain Relief Options

Mounting the USB-621x

Desktop Use

Figure 1-3. Applying Rubber Feet to the USB-621x

DIN Rail Mounting

Panel Mounting

Figure 1-4. Mounting the USB-621x on a Panel

Figure 2-1. Components of a Typical DAQ System

DAQ Hardware

DAQ-STC2

Figure 2-2. USB-621x Block Diagram

Calibration Circuitry

Signal Conditioning

Sensors and Transducers

Cables and Accessories

USB-621x Mass Termination Custom Cabling

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

USB Device Fuse Replacement

Figure 3-1. USB-621x Mass Termination Fuse Location

PWR/ACT LED Indicator

Table 3-2. PWR/ACT LED Status

Figure 4-1. USB-621x Analog Input Circuitry

Analog Input Range

Analog Input Ground-Reference Settings

Figure 4-2. NI-PGIA

Table 4-1. Signals Routed to the NI-PGIA

Configuring AI Ground-Reference Settings in Software

Figure 4-3. Enabling Multimode Scanning in LabVIEW

Multichannel Scanning Considerations

Analog Input Data Acquisition Methods

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