TM
SCXI
SCXI-1581 User Manual
April 2006
323074C-01
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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
the product, refer to the Read Me First: Safety and Radio-Frequency
Interference document, shipped with the product, for precautions to take.
When symbol is marked on a product it denotes a warning advising you to
take precautions to avoid electrical shock.
When symbol is marked on a product it denotes a component that may be
hot. Touching this component may result in bodily injury.
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, and code excerpts.
monospace bold Bold text in this font denotes the messages and responses that the computer
automatically prints to the screen. This font also emphasizes lines of code
that are different from the other examples.
Contents
Chapter 1
About the SCXI-1581
What You Need to Get Started ......................................................................................1-1
National Instruments Documentation ............................................................................1-2
Installing Application Software, NI-DAQ, and the E/M Series DAQ Device ..............1-3
Installing the SCXI-1581 Module into the SCXI Chassis...............................1-4
Verifying the SCXI-1581 Installation............................................................................1-4
Configuring the SCXI System Software .........................................................1-4
Calibrating the SCXI-1581 ............................................................................................1-5
Chapter 2
Connecting Signals
Pin Assignments ............................................................................................................2-1
Chapter 3
Theory of Operation
Scanning Other SCXI Modules Through the SCXI-1581 .............................................3-2
Chapter 4
Using the SCXI-1581
Operation of the Current Sources ..................................................................................4-1
Connecting Resistive Devices to the SCXI-1581 ..........................................................4-1
4-Wire Configuration ......................................................................................4-2
2-Wire Configuration ......................................................................................4-3
3-Wire Resistive Sensor Connected in a 2-Wire Configuration .....................4-4
Lead-Resistance Compensation Using a 3-Wire Resistive Sensor
and Two Matched Current Sources ..............................................................4-5
Lead-Resistance Compensation Using a 3-Wire Resistive Sensor
and Two Differential Amplifiers ..................................................................4-6
Configuring Sensors in Software...................................................................................4-7
Creating an RTD Virtual Channel Using NI-DAQmx ....................................4-7
Creating a Thermistor Virtual Channel Using NI-DAQmx ............................4-8
Measuring Temperature with Resistive Transducers.....................................................4-9
RTDs................................................................................................................4-9
RTD Measurement Errors.................................................................4-9
The Relationship Between Resistance and Temperature
in RTDs ..........................................................................................4-10
© National Instruments Corporation v SCXI-1581 User Manual
Contents
Thermistors ..................................................................................................... 4-13
Thermistor Measurement Circuits .................................................... 4-15
Resistance/Temperature Characteristic of Thermistors.................... 4-16
Appendix A
Specifications
Appendix B
Removing the SCXI-1581
Appendix C
Common Questions
Glossary
Index
Figures
Figure 1-1. SCXI-1581 to DMM Connections ........................................................ 1-4
Figure 3-1. Block Diagram of SCXI-1581 .............................................................. 3-1
Figure 4-1. 4-Wire Resistive Sensor Connected in a 4-Wire Configuration........... 4-2
Figure 4-2. 2-Wire Resistive Sensor Connected in a 2-Wire Configuration........... 4-3
Figure 4-3. 3-Wire Resistive Sensor Connected in a 2-Wire Configuration........... 4-4
Figure 4-4. 3-Wire Configuration Using Matched Current Sources ....................... 4-5
Figure 4-5. 3-Wire Configuration Using Two Differential Amplifiers ................... 4-6
Figure 4-6. 2-Wire RTD Measurement ................................................................... 4-10
Figure 4-7. Resistance-Temperature Curve for a 100 Ω Platinum RTD,
α = 0.00385..........................................................................................4-11
Figure 4-8. Resistance-Temperature Curve for a 2,252 Ω Thermistor.................... 4-14
Figure 4-9. Thermistor Measurement with Constant Current Excitation ................ 4-15
Figure A-1. SCXI-1581 Dimensions........................................................................ A-2
Figure B-1. Removing the SCXI-1581..................................................................... B-2
SCXI-1581 User Manual vi ni.com
Tables
Contents
Table 2-1. Front Signal Pin Assignments ..............................................................2-2
Table 2-2. Signal Descriptions................................................................................2-3
Table 2-3. Rear Signal Pin Assignments ................................................................2-4
Table 2-4. SCXI-1581 Communication Signals .....................................................2-5
Table 4-1. Platinum RTD Types.............................................................................4-12
Table C-1. SCXI-1581 Digital Signals....................................................................C-3
© National Instruments Corporation vii SCXI-1581 User Manual
About the SCXI-1581
The SCXI-1581 module provides 32 channels of 100 µA current excitation.
You can use the SCXI-1581 in any application that requires 100 µA fixed
current excitation. For example you can use the SCXI-1581 to provide
excitation to resistive transducers such as RTDs and thermistors. This
enables other input devices such as the SCXI-1102/B/C to measure the
output of the transducers.
What You Need to Get Started
To set up and use the SCXI-1581, you need the following items:
❑ Hardware
– SCXI-1581 module
– One of the following terminal blocks:
• SCXI-1300
terminal connectivity.
• SCXI-1310—custom kit for custom connectivity.
• BNC-2095—rack-mount terminal block for BNC
connectivity.
• TBX-96—DIN EN mount terminal block with screw terminal
connectivity.
– SCXI or PXI/SCXI combo chassis
– E/M Series DAQ device
– Computer
– Cabling, cable adapter, and sensors as required for your
application
1
1
—front-mount terminal block with screw
1
When connected to an SCXI-1581, you cannot measure the onboard temperature sensor.
© National Instruments Corporation 1-1 SCXI-1581 User Manual
Chapter 1 About the SCXI-1581
❑ Software
– NI-DAQ 7.0 or later
– Application software, such as LabVIEW, LabWindows
Measurement Studio, or other programming environments
❑ Documentation
– Read Me First: Safety and Radio-Frequency Interference
– DAQ Getting Started Guide
– SCXI Quick Start Guide
– SCXI-1581 User Manual
– Documentation for your hardware
– Documentation for your software
❑ Tools
– Wire cutter
– Wire stripper
– Flathead screwdriver
– Phillips screwdriver
™
/CVI™,
National Instruments Documentation
The SCXI-1581 User Manual is one piece of the documentation set for data
acquisition (DAQ) systems. You could have any of several types of
manuals depending on the hardware and software in the system. Use the
manuals you have as follows:
• The SCXI Quick Start Guide—This document contains a quick
overview for setting up an SCXI chassis, installing SCXI modules and
terminal blocks, and attaching sensors. It also describes setting up the
SCXI system in MAX.
• SCXI or PXI/SCXI chassis manual—Read this manual for
maintenance information on the chassis and for installation
instructions.
• The DAQ Getting Started Guide—This document has information on
installing NI-DAQ and the E/M Series DAQ device. Install these
before you install the SCXI module.
• The SCXI hardware user manuals—Read these manuals for detailed
information about signal connections and module configuration. They
SCXI-1581 User Manual 1-2 ni.com
Chapter 1 About the SCXI-1581
also explain, in greater detail, how the module works and contain
application hints.
• Accessory installation guides or manuals—Read the terminal block
and cable assembly installation guides. They explain how to physically
connect the relevant pieces of the system. Consult these guides when
you are making the connections.
• The E/M Series DAQ device documentation—This documentation has
detailed information about the DAQ device that plugs into or is
connected to the computer. Use this documentation for hardware
installation and configuration instructions, specification information
about the DAQ device, and application hints.
• Software documentation—You may have both application software
and NI-DAQ software documentation. National Instruments (NI)
application software includes LabVIEW, LabWindows/CVI, and
Measurement Studio. After you set up the hardware system, use either
your application software documentation or the NI-DAQ
documentation to help you write your application. If you have a large,
complex system, it is worthwhile to look through the software
documentation before you configure the hardware.
• One or more of the following help files for software information:
– Start»Programs»National Instruments»NI-DAQ»
NI-DAQmx Help
– Start»Programs»National Instruments»NI-DAQ»
Traditional NI-DAQ User Manual
– Start»Programs»National Instruments»NI-DAQ»
Traditional NI-DAQ Function Reference Help
You can download NI documents from
the latest version of NI-DAQ, click Download Software at
ni.com/manuals. To download
ni.com.
Installing Application Software, NI-DAQ, and the E/M Series DAQ Device
Refer to the DAQ Getting Started Guide packaged with the NI-DAQ
software to install your application software, NI-DAQ driver software, and
the DAQ device to which you will connect the SCXI-1581. NI-DAQ 7.0 or
later is required to configure and program the SCXI-1581 module. If you
do not have NI-DAQ 7.0 or later, you can either contact an NI sales
representative to request it on a CD or download the latest NI-DAQ version
from
ni.com.
© National Instruments Corporation 1-3 SCXI-1581 User Manual
Chapter 1 About the SCXI-1581
Note Refer to the Read Me First: Safety and Radio-Frequency Interference document
before removing equipment covers or connecting or disconnecting any signal wires.
Installing the SCXI-1581 Module into the SCXI Chassis
Refer to the SCXI Quick Start Guide to install your SCXI-1581 module.
Verifying the SCXI-1581 Installation
The SCXI-1581 has no software configurable settings in MAX for use with
either NI-DAQmx or Traditional NI-DAQ (Legacy).
To verify the functionality of the SCXI-1581 complete the following steps
while referring to Figure 1-1:
1. Connect a high-precision DMM to each Ex (x)+ and Ex (x)– channel,
configured in current measurement mode.
2. Verify that the output is 100 µA and is within the specifications listed
in Appendix A, Specifications.
3. If any channel is not within specifications, ensure that the SCXI
chassis is functioning properly.
4. If the module is still not within specifications, contact NI for further
technical assistance. NI contact information is listed in the Technical
Support Information document.
DMM
Current In
COM/GND
Figure 1-1. SCXI-1581 to DMM Connections
SCXI-1581
Ex (x )+
Ex (x )–
Configuring the SCXI System Software
Refer to the SCXI Quick Start Guide and the user manuals of the modules
in your application to configure and verify them in software.
SCXI-1581 User Manual 1-4 ni.com
Calibrating the SCXI-1581
The SCXI-1581 is within the specifications described in Appendix A,
Specifications, when it is shipped. You can verify that the SCXI-1581 is
within the specification using a DMM of appropriate accuracy for your
application. If a current source on the SCXI-1581 drifts out of specification
over time, a subcomponent has likely failed. If the SCXI-1581 fails to
operate according to the published specifications, send it back to NI for
repair or replacement. For information about contacting NI, refer to the
Technical Support Information document.
Chapter 1 About the SCXI-1581
© National Instruments Corporation 1-5 SCXI-1581 User Manual
Connecting Signals
This chapter discusses signal connections to using the SCXI-1581 module.
Pin Assignments
The pin assignments for the SCXI-1581 front signal connector are shown
in Table 2-1.
Note Do not make any connections to RSVD pins.
2
© National Instruments Corporation 2-1 SCXI-1581 User Manual
Chapter 2 Connecting Signals
Table 2-1. Front Signal Pin Assignments
Front Connector Diagram Pin Number Column A Column B Column C
32 NC EX0– EX0+
Column
A B C
32
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
NC means no connection 2 CGND EX30– EX30+
RSVD means reserved 1 RSVD EX31– EX31+
31 NC EX1– EX1+
30 NC EX2– EX2+
29 NC EX3– EX3+
28 RSVD EX4– EX4+
27 RSVD EX5– EX5+
26 RSVD EX6– EX6+
25 RSVD EX7– EX7+
24 NC EX8– EX8+
23 NC EX9– EX9+
22 NC EX10– EX10+
21 NC EX11– EX11+
20 RSVD EX12– EX12+
19 RSVD EX13– EX13+
18 NC EX14– EX14+
17 NC EX15– EX15+
16 NC EX16– EX16+
15 NC EX17– EX17+
14 NC EX18– EX18+
13 NC EX19– EX19+
12 NC EX20– EX20+
11 NC EX21– EX21+
10 NC EX22– EX22+
9 NC EX23– EX23+
8 NC EX24– EX24+
7 NC EX25– EX25+
6 NC EX26– EX26+
5 NC EX27– EX27+
4 NC EX28– EX28+
3 NC EX29– EX29+
SCXI-1581 User Manual 2-2 ni.com
Chapter 2 Connecting Signals
Table 2-2. Signal Descriptions
Pin Signal Name Description
A1, A19, A20,
A25–28
RSVD Reserved—this pin is reserved. Do not connect
any signal to this pin.
A2 CGND Chassis Ground—connects to the SCXI chassis.
B1–32 EX<0..31>– Negative Excitation—connects to the channel
ground reference. This is the return path for the
corresponding EX+ channel.
C1–32 EX<0..31>+ Positive excitation—connects to the positive
current output of the channel.
The rear signal connector, shown in Table 2-3, is used for analog signal
connectivity and communication between the SCXI-1581 and the
E/M Series DAQ device. Grounding signals AIGND and OUTREF provide
reference signals needed in the various analog input referencing modes on
the E/M Series DAQ device. In multiplexed mode, the CH0 signal pair is
used for sending analog signals from other modules to the connected
E/M Series DAQ device. If the module is directly connected to the
E/M Series DAQ device, the other analog channels of the E/M Series DAQ
device are available for general-purpose analog input because they are not
connected to the SCXI-1581 in multiplexed mode.
The communication signals between the E/M Series DAQ device and the
SCXI system are SERDATIN, SERDATOUT, DAQD*/A, SLOT0SEL*,
SERCLK, and SCANCLK. The digital ground, DIGGND on pins
24 and 33, provides a separate ground reference for the communication
signals. SERDATIN, SERDATOUT, DAQD*/A, SLOT0SEL*, and
SERCLK are the communication lines for programming the SCXI-1581.
The SCANCLK and SYNC signals are the signals necessary for
multiplexed mode scanning. If the E/M Series DAQ device is connected to
the SCXI-1581, these digital lines are unavailable for general-purpose
digital I/O.
© National Instruments Corporation 2-3 SCXI-1581 User Manual
Chapter 2 Connecting Signals
Rear Connector
Diagram
12
34
56
78
910
11 12
13 14
15 16
17 18
19 20
21 22
23 24
25 26
27 28
29 30
31 32
33 34
35 36
37 38
39 40
41 42
43 44
45 46
47 48
49 50
Table 2-3. Rear Signal Pin Assignments
Signal Name Pin Number Pin Number Signal Name
AI GND 1 2 AI GND
CH 0 + 3 4 CH 0 –
— 5 6 —
— 7 8 —
— 9 10 —
— 11 12 —
— 13 14 —
— 15 16 —
— 17 18 —
OUT REF 19 20 —
— 21 22 —
— 23 24 DIG GND
SER DAT IN 25 26 SER DAT OUT
DAQ D*/A 27 28 —
SLOT 0 SEL* 29 30 —
— 31 32 —
DIG GND 33 34 —
— 35 36 AI HOLD COMP,
AI HOLD
SER CLK 37 38 —
— 39 40 —
— 41 42 —
— 43 44 —
— 45 46 SYNC
— 47 48 —
— 49 50 —
SCXI-1581 User Manual 2-4 ni.com
Chapter 2 Connecting Signals
The communication signals between the DAQ device and the SCXI system
are listed in Table 2-4. If the DAQ device is connected to the SCXI-1581,
these digital lines are unavailable for general-purpose digital I/O.
Table 2-4. SCXI-1581 Communication Signals
NI-DAQmx
SCXI
Pin
24, 33 DIG GND D GND DGND — Digital ground—these
25 SER DAT IN P0.0 DIO0 Input Serial data in—this
26 SER DAT OUT P0.4 DIO4 Output Serial data out—this
27 DAQ D*/A P0.1 DIO1 Input Board data/address
29 SLOT 0 SEL* P0.2 DIO2 Input Slot 0 select—this
Signal Name
Device Signal
Name
Traditional NI-DAQ
(Legacy) Device
Signal Name
Direction Description
pins supply the
reference for
E/M Series DAQ device
digital signals and are
connected to the
module digital ground.
signal taps into the
SCXIbus MOSI line to
send serial input data to
a module or Slot 0.
signal taps into the
SCXIbus MISO line to
accept serial output data
from a module.
line—this signal taps
into the SCXIbus D*/A
line to indicate to the
module whether the
incoming serial stream
is data or address
information.
signal taps into the
SCXIbus INTR* line to
indicate whether the
information on MOSI is
being sent to a module
or Slot 0.
© National Instruments Corporation 2-5 SCXI-1581 User Manual
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