NATIONAL INSTRUMENTS SCXI-1581 Worldwide Technical Support User Manual

NATIONAL INSTRUMENTS SCXI-1581 Worldwide Technical Support

Product Information

Specifications:

• Model: SCXI-1581
• Brand: SCXI
• Year: April 2006

Product Usage Instructions

Warranty Information:

The SCXI-1581 is warranted against defects in materials and workmanship for a period of one year from the date of shipment.
National Instruments will repair or replace defective equipment during the warranty period, covering parts and labor.

Copyright:

This publication may not be reproduced or transmitted without the prior written consent of National Instruments Corporation. Respect intellectual property rights when using NI software.

Trademarks:

National Instruments, NI, ni.com, and LabVIEW are trademarks of National Instruments Corporation. Refer to the Terms of Use section on ni.com/legal for more information about NI trademarks.

Patents:

WARNING: National Instruments products are not designed for use in surgical implants or critical components in life support systems.

FAQs

• Q: What is the warranty period for SCXI-1581?
  • A: The warranty period is one year from the date of shipment.
• Q: Can I reproduce or transmit the user manual?
  • A: No, you need prior written consent from National Instruments Corporation to reproduce or transmit the manual.

SCXI-1581

SCXITM
SCXI-1581 User Manual
April 2006 323074C-01

Worldwide Technical Support and Product Information ni.com
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For further support information, refer to the Signal Conditioning Technical Support Information document. To comment on National Instruments documentation, refer to the National Instruments Web site at ni.com/info and enter the info code feedback.
© 2001­2006 National Instruments Corporation. All rights reserved.

Important Information

Warranty
The SCXI-1581 is 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 during the warranty period. This warranty includes parts and labor.
The media on which you receive National Instruments software are warranted not to fail to execute programming instructions, due to defects in materials and workmanship, for a period of 90 days from date of shipment, as evidenced by receipts or other documentation. National Instruments will, at its option, repair or replace software media that do not execute programming 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 consult National Instruments if errors are suspected. In no event shall National Instruments be liable for any damages arising out of or related to this document or the information contained in it.
EXCEPT AS SPECIFIED HEREIN, NATIONAL INSTRUMENTS MAKES NO WARRANTIES, EXPRESS OR IMPLIED, AND SPECIFICALLY DISCLAIMS ANY WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. CUSTOMER’S RIGHT TO RECOVER DAMAGES CAUSED BY FAULT OR NEGLIGENCE ON THE PART OF NATIONAL INSTRUMENTS SHALL BE LIMITED TO THE AMOUNT THERETOFORE PAID BY THE CUSTOMER. NATIONAL INSTRUMENTS WILL NOT BE LIABLE FOR DAMAGES RESULTING FROM LOSS OF DATA, PROFITS, USE OF PRODUCTS, OR INCIDENTAL OR CONSEQUENTIAL DAMAGES, EVEN IF ADVISED OF THE POSSIBILITY THEREOF. This limitation of the liability of National Instruments will apply regardless of the form of action, whether in contract or tort, including negligence. Any action against National Instruments must be brought within one year after the cause of action accrues. National Instruments shall not be liable for any delay in performance due to causes beyond its reasonable control. The warranty provided herein does not cover damages, defects, malfunctions, or service failures caused by owner’s failure to follow the National Instruments installation, operation, or maintenance instructions; owner’s modification of the product; owner’s abuse, misuse, or negligent acts; and power failure or surges, fire, flood, accident, actions of third parties, or other events outside reasonable control.
Copyright
Under the copyright laws, this publication may not be reproduced or transmitted in any form, electronic or mechanical, including photocopying, recording, storing in an information retrieval system, or translating, in whole or in part, without the prior written consent of National Instruments Corporation.
National Instruments respects the intellectual property of others, and we ask our users to do the same. NI software is protected by copyright and other intellectual property laws. Where NI software may be used to reproduce software or other materials belonging to others, you may use NI software only to reproduce materials that you may reproduce in accordance with the terms of any applicable license or other legal restriction.
Trademarks
National Instruments, NI, ni.com, and LabVIEW are trademarks of National Instruments Corporation. Refer to the Terms of Use section on ni.com/legal for more information about National Instruments trademarks.
Other product and company names mentioned herein are trademarks or trade names of their respective companies.
Patents
For patents covering National Instruments products, refer to the appropriate location: Help»Patents in your software, the patents.txt file on your CD, or ni.com/patents.
WARNING REGARDING USE OF NATIONAL INSTRUMENTS PRODUCTS
(1) NATIONAL INSTRUMENTS PRODUCTS ARE NOT DESIGNED WITH COMPONENTS AND TESTING FOR A LEVEL OF RELIABILITY SUITABLE FOR USE IN OR IN CONNECTION WITH SURGICAL IMPLANTS OR AS CRITICAL COMPONENTS IN ANY LIFE SUPPORT SYSTEMS WHOSE FAILURE TO PERFORM CAN REASONABLY BE EXPECTED TO CAUSE SIGNIFICANT INJURY TO A HUMAN.
(2) IN ANY APPLICATION, INCLUDING THE ABOVE, RELIABILITY OF OPERATION OF THE SOFTWARE PRODUCTS CAN BE IMPAIRED BY ADVERSE FACTORS, INCLUDING BUT NOT LIMITED TO FLUCTUATIONS IN ELECTRICAL POWER SUPPLY, COMPUTER HARDWARE MALFUNCTIONS, COMPUTER OPERATING SYSTEM SOFTWARE FITNESS, FITNESS OF COMPILERS AND DEVELOPMENT SOFTWARE USED TO DEVELOP AN APPLICATION, INSTALLATION ERRORS, SOFTWARE AND HARDWARE COMPATIBILITY PROBLEMS, MALFUNCTIONS OR FAILURES OF ELECTRONIC MONITORING OR CONTROL DEVICES, TRANSIENT FAILURES OF ELECTRONIC SYSTEMS (HARDWARE AND/OR SOFTWARE), UNANTICIPATED USES OR MISUSES, OR ERRORS ON THE PART OF THE USER OR APPLICATIONS DESIGNER (ADVERSE FACTORS SUCH AS THESE ARE HEREAFTER COLLECTIVELY TERMED “SYSTEM FAILURES”). ANY APPLICATION WHERE A SYSTEM FAILURE WOULD CREATE A RISK OF HARM TO PROPERTY OR PERSONS (INCLUDING THE RISK OF BODILY INJURY AND DEATH) SHOULD NOT BE RELIANT SOLELY UPON ONE FORM OF ELECTRONIC SYSTEM DUE TO THE RISK OF SYSTEM FAILURE. TO AVOID DAMAGE, INJURY, OR DEATH, THE USER OR APPLICATION DESIGNER MUST TAKE REASONABLY PRUDENT STEPS TO PROTECT AGAINST SYSTEM FAILURES, INCLUDING BUT NOT LIMITED TO BACK-UP OR SHUT DOWN MECHANISMS. BECAUSE EACH END-USER SYSTEM IS CUSTOMIZED AND DIFFERS FROM NATIONAL INSTRUMENTS’ TESTING PLATFORMS AND BECAUSE A USER OR APPLICATION DESIGNER MAY USE NATIONAL INSTRUMENTS PRODUCTS IN COMBINATION WITH OTHER PRODUCTS IN A MANNER NOT EVALUATED OR CONTEMPLATED BY NATIONAL INSTRUMENTS, THE USER OR APPLICATION DESIGNER IS ULTIMATELY RESPONSIBLE FOR VERIFYING AND VALIDATING THE SUITABILITY OF NATIONAL INSTRUMENTS PRODUCTS WHENEVER NATIONAL INSTRUMENTS PRODUCTS ARE INCORPORATED IN A SYSTEM OR APPLICATION, INCLUDING, WITHOUT LIMITATION, THE APPROPRIATE DESIGN, PROCESS AND SAFETY LEVEL OF SUCH SYSTEM OR APPLICATION.

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 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 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.
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.
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.

1 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-13001–front-mount terminal block with screw 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 When connected to an SCXI-1581, you cannot measure the onboard temperature sensor.

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About the SCXI-1581

Software
­ NI-DAQ 7.0 or later ­ Application software, such as LabVIEW, LabWindowsTM/CVITM,
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
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

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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 ni.com/manuals. To download the latest version of NI-DAQ, click Download Software at 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.

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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

SCXI-1581 Ex (x )+

COM/GND

Ex (x )­

Figure 1-1. SCXI-1581 to DMM Connections
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.

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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.

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2
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.

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Front Connector Diagram
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
RSVD means reserved

Table 2-1. Front Signal Pin Assignments

Pin Number 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

Column A NC NC NC NC
RSVD RSVD RSVD RSVD
NC NC NC NC RSVD RSVD NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC CGND RSVD

Column B EX0­ EX1­ EX2­ EX3­ EX4­ EX5­ EX6­ EX7­ EX8­ EX9­ EX10­ EX11­ EX12­ EX13­ EX14­ EX15­ EX16­ EX17­ EX18­ EX19­ EX20­ EX21­ EX22­ EX23­ EX24­ EX25­ EX26­ EX27­ EX28­ EX29­ EX30­ EX31­

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Column C EX0+ EX1+ EX2+ EX3+ EX4+ EX5+ EX6+ EX7+ EX8+ EX9+ EX10+ EX11+ EX12+ EX13+ EX14+ EX15+ EX16+ EX17+ EX18+ EX19+ EX20+ EX21+ EX22+ EX23+ EX24+ EX25+ EX26+ EX27+ EX28+ EX29+ EX30+ EX31+
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Pin A1, A19, A20, A25­28 A2 B1­32
C1­32

Chapter 2 Connecting Signals

Table 2-2. Signal Descriptions

Signal Name

Description

RSVD

Reserved–this pin is reserved. Do not connect any signal to this pin.

CGND

Chassis Ground–connects to the SCXI chassis.

EX<0..31> ­

Negative Excitation–connects to the channel ground reference. This is the return path for the corresponding EX+ channel.

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.

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Rear Connector Diagram
12 34 56 78 9 10 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 AI GND CH 0 + — — — — — — — OUT REF — —
SER DAT IN DAQ D/A SLOT 0 SEL
— DIG GND
—
SER CLK — — — — — —

Pin Number 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35

Pin Number 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

Signal Name AI GND CH 0 ­ — — — — — — — — — DIG GND
SER DAT OUT — — — —
AI HOLD COMP, AI HOLD — — — — SYNC — —

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Pin 24, 33
25 26 27
29

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

SCXI Signal Name
DIG GND
SER DAT IN SER DAT OUT
DAQ D/A
SLOT 0 SEL

NI-DAQmx Device Signal
Name D GND
P0.0
P0.4
P0.1
P0.2

Traditional NI-DAQ (Legacy) Device Signal Name DGND
DIO0
DIO4
DIO1
DIO2

Direction —
Input Output Input
Input

Description
Digital ground–these pins supply the reference for E/M Series DAQ device digital signals and are connected to the module digital ground.
Serial data in–this signal taps into the SCXIbus MOSI line to send serial input data to a module or Slot 0.
Serial data out–this signal taps into the SCXIbus MISO line to accept serial output data from a module.
Board data/address 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.
Slot 0 select–this signal taps into the SCXIbus INTR line to indicate whether the information on MOSI is being sent to a module or Slot 0.

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Table 2-4. SCXI-1581 Communication Signals (Continued)

NI-DAQmx

Traditional NI-DAQ

SCXI

Device Signal

(Legacy) Device

Pin

Signal Name

Name

Signal Name

Direction

Description

36

SCAN CLK AI HOLD COMP,

AI HOLD

SCANCLK

Input

Scan clock–a rising edge indicates to the scanned SCXI module that the E/M Series DAQ device has taken a sample and causes the module to advance channels.

37

SER CLK

EXT STROBE*

EXTSTROBE*

Input

Serial clock–this signal taps into the SCXIbus SPICLK line to clock the data on the MOSI and MISO lines.

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3
Theory of Operation

This chapter provides a brief overview and a detailed discussion of the circuit features of the SCXI-1581 module. Refer to Figure 3-1 while reading this section.

Rear Signal Connector

SCXI-1300 Terminal Block
CH 0 +
CH 0 ­
Screw Terminals
to Field Wiring

SCXI-1581 Module 100

Buffer

CH 0 + CH 0 ­

To Analog
Bus
Analog Bus Switch

Scan Clock
AB 0 + AB 0 ­

Digital Interface and Control

SCXIbus Connector

CH31 + 100
CH31 ­

Figure 3-1. Block Diagram of SCXI-1581

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Chapter 3

Theory of Operation

Scanning Other SCXI Modules Through the SCXI-1581
When connected as the cabled module in an SCXI chassis, the SCXI-1581 can route the multiplexed signals from other SCXI modules to the E/M Series DAQ device. The SCXI-1581 routes multiplexed signals from other SCXI modules to the E/M Series DAQ device even though the SCXI-1581 does not have any analog- input channels. Refer to the user manuals for your other SCXI modules for details about scanning those modules in multiplexed mode.

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4
Using the SCXI-1581
This chapter makes suggestions for developing your application.
Operation of the Current Sources
The current sources on the SCXI-1581 continuously provide 32 channels of 100 µA current excitation. These current sources are on whenever the SCXI chassis is powered-on. The current sources on the SCXI-1581 are designed to be accurate to within ±0.05% of the specified value with a temperature drift of no more than ±5 ppm/°C. The high accuracy and stability of these current sources makes them especially well suited to measuring resistance to a high degree of accuracy.
Connecting Resistive Devices to the SCXI-1581
You can connect resistive devices to the SCXI signal conditioning system in a 4-, 2-, or 3-wire configuration. The SCXI-1102/B/C modules are 32-channel analog-input modules that are ideally suited for measuring DC or slowly varying voltages. Figures 4-1 through 4-5 illustrate various ways to connect sensors for current excitation and voltage measurements using the SCXI-1581 and the SCXI-1102B/C modules.
Refer to the appropriate ADE and SCXI documentation for information concerning setting appropriate voltage gains for the analog inputs.
You can use the SCXI-1300 terminal block to make signal connections to the SCXI-1581 and SCXI-1102 modules. When using the SCXI-1300 terminal block, terminals EX<0..31>+ and EX<0..31>­ map to terminals CH<0..31>+ and CH<0..31>­ respectively on the SCXI-1300 terminal block.

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Using the SCXI-1581

4-Wire Configuration
The 4-wire configuration, also referred to as a Kelvin connection, is shown in Figure 4-1. The 4-wire configuration uses one pair of wires to deliver the excitation current to the resistive sensor and uses a separate pair of wires to sense the voltage across the resistive sensor. Because of the high input impedance of the differential amplifier, negligible current flows through the sense wires. This results in a very small lead-resistance voltage drop error. The main disadvantage of the 4-wire connection is the greater number of field wires required.

RL1 RL2 RT
RL3

SCXI-1300 EX0+

SCXI-1581

EX0­

SCXI-1300

RL4

CH0+

RL1, RL2, RL3, and RL4 are not required to be equal

CH0­

­ +

SCXI-1102

Figure 4-1. 4-Wire Resistive Sensor Connected in a 4-Wire Configuration

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Chapter 4 Using the SCXI-1581
2-Wire Configuration
The basic 2-wire configuration is shown in Figure 4-2. In this configuration an error voltage (VE) is introduced into the measurement equal to the excitation current (IEX) times the sum of the two lead resistances, RL1 and RL2. If we assume equal lead resistances, RL1 = RL2 = RL, the magnitude of the error voltage is:
VE = 2RLIEX
This is the most commonly used configuration for connecting thermistors to a signal conditioning system because the large sensitivity of thermistors usually results in the introduction of a negligible error by the lead resistances.
RTDs typically have a much smaller sensitivity and nominal resistance than thermistors, therefore a 2-wire configuration usually results in the introduction of larger errors by the lead resistance.

RL1
RT RL2

SCXI-1300 EX0+

SCXI-1581

EX0­

Add These Connections

SCXI-1300 CH0+

CH0­

In this configuration, the lead resistance due to RL1 and RL2 can introduce measurement error.

­ +

SCXI-1102

Figure 4-2. 2-Wire Resistive Sensor Connected in a 2-Wire Configuration

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3-Wire Resistive Sensor Connected in a 2-Wire Configuration
If you are using a 3-wire resistive sensor, you can reduce the error voltage by one-half over the 2-wire measurement by connecting the device as shown in Figure 4-3. In this configuration, very little current flows through RL3 and therefore RL1 is the only lead resistance that introduces an error into the measurement. The resulting measurement error is:
VE = RL1IEX
An advantage of this configuration is that it only requires a single jumper wire from the SCXI-1581 EX0+ terminal to the SCXI-1102B/C CH0+ terminal.

RL1
RT RL2

SCXI-1300 EX0+

SCXI-1581

EX0­

Add This Connection

SCXI-1300

RL3

CH0+

CH0­

In this configuration, the lead resistance due to RL1 can introduce measurement error.

­ +

SCXI-1102

Figure 4-3. 3-Wire Resistive Sensor Connected in a 2-Wire Configuration

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Chapter 4 Using the SCXI-1581
Lead-Resistance Compensation Using a 3-Wire Resistive Sensor and Two Matched Current Sources
You can compensate for the errors introduced by lead-resistance voltage drops by using a 3-wire resistive sensor and two matched current sources connected as shown in Figure 4-4.

Assume

RL1

RL1 = RL2

RT RL2

RL3

Add These Connections

SCXI-1300 EX0+ EX1+

SCXI-1581

EX0­

SCXI-1300
CH0+ CH0­

­ +

SCXI-1102

Figure 4-4. 3-Wire Configuration Using Matched Current Sources
In this configuration, the lead-resistance voltage drop across RL3 is converted into a common-mode voltage that is rejected by the differential amplifier. Also, the polarity of the lead-resistance voltage drops across RL1 and RL2 are series opposing, relative to the inputs of the differential amplifier, eliminating their effect on the voltage measured across RT.
Note RL1 and RL2 are assumed to be equal.
The effectiveness of this method depends on the matching of the current sources. Each current source on the SCXI-1581 has an accuracy of ±0.05%. This accuracy results in a worst-case matching of ±0.1%. Refer to the Measuring Temperature with Resistive Transducers section for accuracy considerations of RTDs and thermistors.

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Chapter 4 Using the SCXI-1581
Lead-Resistance Compensation Using a 3-Wire Resistive Sensor and Two Differential Amplifiers
If the accuracy obtained by using a 3-wire device and matched current sources is not sufficient for your application, you can eliminate the error due to the mismatch of the current sources by using only one current source and two differential amplifiers. The 3-wire, 2-amplifier configuration is illustrated in Figure 4-5.

RL1

SCXI-1300

SCXI-1581

EX0+

RT

RL2

EX0­

Add These Connections

­ + ­ +

SCXI-1300

SCXI-1102

RL3

CH0+

CH0­

V1 = VRL1 + VRT

CH1+ CH1­

V2 = VRL2

Figure 4-5. 3-Wire Configuration Using Two Differential Amplifiers
In this configuration, two separate measurements are taken; the first, labeled V1 in Figure 4-5, is the sum of the voltage drops across the lead resistance RL1 and the resistive device RT. If the voltage drop across RL1 and RT is denoted as VRL1 and VRT respectively, the expression for V1 becomes:
V1 = VRL1 + VRT

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The second measurement, labeled V2 in Figure 4-5, is equal to the voltage drop across the lead resistance RL2, denoted as VRL2; therefore:
V2 = VRL2
If the lead resistances RL1 and RL2 are assumed equal, you can remove in software the error voltages due to the lead resistances by subtracting V2 from V1. In most 3-wire device applications the lead wires are all the same length and made of the same material, therefore substantiating the assumption of equal lead resistances.
Configuring Sensors in Software
You can create a virtual channel to convert RTD voltages into temperature readings. To create an RTD virtual channel, refer to the Creating an RTD Virtual Channel Using NI-DAQmx section.
Creating an RTD Virtual Channel Using NI-DAQmx
To create an RTD virtual channel using NI-DAQmx, complete the following steps:

1. Launch MAX. 2. Right-click Data Neighborhood and select Create New. 3. Select NI-DAQmx Global Virtual Channel and click Next. 4. Select Analog Input»Temperature»RTD. 5. Select the analog input device and channel to use, and click Next. 6. Enter a name for the virtual channel and click Finish. 7. In the configuration window that opens, set the following properties:
   · Signal input range–set the min and max to correspond to the measurement range of your application in terms of units that you select under Scaled Units.
   · RTD type–refer to Table 4-1 for a list of RTD types. · R0–the nominal resistance value of the RTD. · Configuration–how the RTD is connected. · Iex source–select External when connected to an SCXI-1581. · Iex value (A)–100 µ when connected to the SCXI-1581.

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8. Click the device tab and set any device specific properties that are applicable for the measurement device.
9. To test the NI-DAQmx RTD virtual channel, click the Test button.
You have finished creating the NI-DAQmx virtual channel. You can access the channel by expanding Data Neighborhood»NI-DAQmx Channels.
For more information about incorporating the virtual channel into a task with the application, refer to the user manual of the analog input device to which the sensor connects.
Creating a Thermistor Virtual Channel Using NI-DAQmx
To create an thermistor virtual channel using NI-DAQmx, complete the following steps: 1. Launch MAX. 2. Right-click Data Neighborhood and select Create New. 3. Select NI-DAQmx Global Virtual Channel and click Next. 4. Select Analog Input»Temperature»Iex Thermistor. 5. Select the analog input device and channel to which the sensor
connects, and click Next. 6. Enter a name for the virtual channel and click Finish. 7. In the configuration window that opens, set the following properties:
· Signal input range–set the min and max to correspond to the measurement range of your application in terms of units that you select under Scaled Units.
· A, B, C–these scaling coefficients are obtained from the thermistor manufacturer, or calculated resistance-versus-temperature curves.
· Iex source–select External when connected to an SCXI-1581 · Iex value (A)–100 µ when connected to the SCXI-1581. · Configuration–the wire configuration of the sensor. 8. Click the device tab and set any device specific properties that are applicable for the measurement device. 9. To test the NI-DAQmx thermistor virtual channel, click the Test button.
You have finished creating the NI-DAQmx virtual channel. You can access the channel by expanding Data Neighborhood»NI-DAQmx Channels.

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For more information about incorporating the virtual channel into a task with the application, refer to the user manual of the analog input device to which the sensor connects.

Measuring Temperature with Resistive Transducers

This section discusses RTDs and thermistors, and describes accuracy considerations when connecting resistive transducers to the signal conditioning system.

RTDs

A resistive-temperature detector (RTD) is a temperature-sensing device whose resistance increases with temperature. An RTD consists of a wire coil or deposited film of pure metal. RTDs are made of different metals and have different resistances, but the most popular RTD is made of platinum and has a nominal resistance of 100 at 0 °C.
RTDs are known for their excellent accuracy over a wide temperature range. Some RTDs have accuracies as high as 0.01 (0.026 °C) at 0 °C. RTDs are also extremely stable devices. Common industrial RTDs drift less than 0.1 °C/year, and some models are stable to within 0.0025 °C/year.
RTDs are sometimes difficult to measure because they have relatively low nominal resistance (commonly 100 ) that changes only slightly with temperature (less than 0.4 /°C). To accurately measure these small changes in resistance, you must use special configurations that minimize measured errors caused by lead-wire resistance.

RTD Measurement Errors
Because the RTD is a resistive device, you must pass a current through the device and monitor the resulting voltage. However, any resistance in the lead wires that connect the measurement system to the RTD adds error to the readings. For example, consider a 2-wire RTD element connected to a measurement system that also supplies a constant current, IEX, to excite the RTD. As shown in Figure 4-6, the voltage drop across the lead resistances (labeled RL) adds an error voltage to the measured voltage.

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IEX RL

V0 ­

RT

RL

Figure 4-6. 2-Wire RTD Measurement
The maximum resistance of the thermistor is determined from the current excitation value and the maximum voltage range of the input device. When using the SCXI-1581 with an SCXI-1102/B/C, the maximum measurable resistance is 100 k. Refer to Appendix A, Specifications, for the maximum ratings.
For example, a lead resistance of 0.3 in each wire adds a 0.6 error to the resistance measurement. For a platinum RTD at 0 °C with = 0.00385, the lead resistance equates to an error of approximately
0—-.-3—80—5.–6——–/–°—C– = 1.6 °C
The Connecting Resistive Devices to the SCXI-1581 section describes different ways of connecting resistive devices to the SCXI system.
The Relationship Between Resistance and Temperature in RTDs
Compared to other temperature-measurement devices, the output of an RTD is relatively linear with respect to temperature. The temperature coefficient, called alpha (), differs between RTD curves. Although various manufacturers specify alpha differently, alpha is most commonly defined as the change in RTD resistance from 0 to 100 °C, divided by the resistance at 0 °C, divided by 100 °C:
( / / (°C)) = R—-R0—-×1–0–0-1—0­—0–R—°0–C—

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where R100 is the resistance of the RTD at 100 °C. R0 is the resistance of the RTD at 0 °C.
For example, a 100 platinum RTD with = 0.003911 has a resistance of 139.11 at 100 °C. Figure 4-7 displays a typical resistance-temperature curve for a 100 platinum RTD.
480
400
320
240
160
80 0 80 160 240 320 400 480 560 640 720 800 880 960
Figure 4-7. Resistance-Temperature Curve for a 100 Platinum RTD, = 0.00385 Although the resistance-temperature curve is relatively linear, accurately converting measured resistance to temperature requires curve fitting. The following Callendar-Van Dusen equation is commonly used to approximate the RTD curve:
RT = R0[1 + AT + BT2 + C(T ­ 100)3]

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where RT is the resistance of the RTD at temperature T. R0 is the resistance of the RTD at 0 °C. A, B, and C are the Callendar-Van Dusen coefficients shown in Table 4-1. T is the temperature in °C.

Table 4-1 lists the RTD types and their corresponding coefficients.

Table 4-1. Platinum RTD Types

Standard
IEC-751 DIN 43760 BS 1904 ASTM-E1137 EN-60751
Low cost vendor compliant1

Temperature Coefficient of Resistance (TCR, PPM) 3851
3750

JISC 1604

3916

US Industrial Standard D-100 American
US Industrial Standard American
ITS-90

3920 3911 3928

1 No standard. Check TCR.

Typical R0
100 1000

Callendar-Van Dusen Coefficient
A = 3.9083 × 10­3 B = ­5.775 × 10­7 C = ­4.183 × 10­12

1000 100 100 100 100

A = 3.81 × 10­3 B = ­6.02 × 10­7 C = ­6.0 × 10­12 A = 3.9739 × 10­3 B = ­5.870 × 10­7 C = ­4.4 × 10­12 A = 3.9787 × 10­3 B = ­5.8686 × 10­7 C = ­4.167 × 10­12 A = 3.9692 × 10­3 B = ­5.8495 × 10­7 C = ­4.233 × 10­12 A = 3.9888 × 10­3 B = ­5.915 × 10­7 C = ­3.85 × 10­12

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Chapter 4 Using the SCXI-1581

Thermistors

For temperatures above 0 °C, coefficient C equals 0, reducing this equation
to a quadratic. If you pass a known current, IEX, through the RTD and measure the output voltage developed across the RTD, V0, you can solve for T as follows:

T = —————————————R—-0—­—–I–V—E—–0-X—————————————–

­0.5

R0

A

R02A2

­

4

R

0B

R0

­

I-V–E–0-X-

where V0 is the measured RTD voltage. IEX is the excitation current.

A thermistor is a piece of semiconductor made from metal oxides, pressed into a small bead, disk, wafer, or other shape, sintered at high temperatures, and finally coated with epoxy or glass. The resulting device exhibits an electrical resistance that varies with temperature.
There are two types of thermistors: negative temperature coefficient (NTC) thermistors, whose resistance decreases with increasing temperature, and positive temperature coefficient (PTC) thermistors, whose resistance increases with increasing temperature. NTC thermistors are more commonly used than PTC thermistors, especially for temperature measurement applications.
A main advantage of thermistors for temperature measurement is their extremely high sensitivity. For example, a 2,252 thermistor has a sensitivity of ­100 /°C at room temperature. Higher resistance thermistors can exhibit temperature coefficients of ­10 k/°C or more. In comparison, a 100 platinum RTD has a sensitivity of only 0.4 /°C.
Also, the physically small size and low thermal mass of a thermistor bead allows a very fast response to temperature changes.
Another advantage of the thermistor is its relatively high resistance. Thermistors are available with base resistances (at 25 °C) ranging from hundreds to millions of ohms. This high resistance diminishes the effect of inherent resistances in the lead wires, which can cause significant errors with low resistance devices such as RTDs. For example, while RTD measurements typically require 3- or 4-wire connections to reduce errors

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Resistance ()

caused by lead-wire resistances, 2-wire connections to thermistors are usually adequate.
The major trade-off for the high resistance and sensitivity of the thermistor is its highly nonlinear output and relatively limited operating range. Depending on the type of thermistor, the upper range is typically limited to around 300 °C. Figure 4-8 shows the resistance-temperature curve for a 2,252 thermistor. The curve of a 100 RTD is also shown for comparison.

10 M

1 M 100 k

Thermistor (2,252 at 25 °C)

10 k

1 k

RTD

(PT 100 )

100

10

1 ­200 ­150 ­100 ­50 0

50 100 150 200 250 300 350 400 Temperature (°C)

Figure 4-8. Resistance-Temperature Curve for a 2,252 Thermistor

The thermistor has been used primarily for high-resolution measurements over limited temperature ranges. However, continuing improvements in thermistor stability, accuracy, and interchangeability have prompted increased use of thermistors in a variety of applications.

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Chapter 4 Using the SCXI-1581
Thermistor Measurement Circuits
This section details information about thermistor measurement circuits. The most common technique is to use a current-source, and measure the voltage developed across the thermistor. Figure 4-9 shows the measured voltage V0 equals IEX × RT.

IEX
+ V0
­

RT
Thermistor

V0 = IEX x RT
Figure 4-9. Thermistor Measurement with Constant Current Excitation
The level of the voltage output signal depends directly on the thermistor resistance and magnitude of the current excitation. Do not use a higher level of current excitation in order to produce a higher level output signal because the current causes the thermistor to heat internally, leading to temperature- measurement errors. This phenomena is called self-heating. When current passes through the thermistor, power dissipated by the thermistor equaling (IEX2RT), heats the thermistor.
Thermistors, with their small size and high resistance, are particularly prone to these self-heating errors. Manufacturers typically specify this self- heating as a dissipation constant, which is the power required to heat the thermistor 1 °C from ambient temperature (mW/°C). The dissipation constant depends heavily on how easily heat is transferred away from the thermistor, so the dissipation constant can be specified for different media–in still air, water, or oil bath. Typical dissipation constants range anywhere from less than 0.5 mW/°C for still air to 10 mW/°C or higher for a thermistor immersed in water. A 2,252 thermistor powered by a 100 µA excitation current dissipates:
I2R = 100 µA2 × 2,252 = 0.0225 mW
If this thermistor has a dissipation constant of 10 mW/°C, the thermistor self-heats 0.00225 °C so the self-heating from the 100 µA source of the

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SCXI-1581 is negligible for most applications. It is still important to carefully read self-heating specifications of the thermistors.
Resistance/Temperature Characteristic of Thermistors
The resistance-temperature behavior of thermistors is highly dependent upon the manufacturing process. Therefore, thermistor curves are not standardized to the extent that thermocouple or RTD curves are standardized. Typically, thermistor manufacturers supply the resistance-versus-temperature curves or tables for their particular devices. You can, however, approximate the thermistor curve relatively accurately with the Steinhart-Hart equation:
T(°K ) = ——————————–1——————————–a + b[ln(RT)] + c[ln(RT)]3
where T(°K) is the temperature in degrees Kelvin, equal to T(°C) + 273.15. RT is the resistance of the thermistor. a, b, and c are coefficients obtained from the thermistor manufacturer or calculated from the resistance-versus- temperature curve.

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Specifications

This appendix lists the specifications for the SCXI-1581 modules. These specifications are typical at 25 °C unless otherwise noted.

Stability

Recommended warm-up time ……………. 10 minutes

Excitation

Channels…………………………………………. 32 single-ended outputs Current output …………………………………. 100 µA Accuracy ………………………………………… ±0.05% Temperature drift …………………………….. ±5 ppm/°C Output voltage compliance ……………….. 10 V Maximum resistive load……………………. 100 k Overvoltage protection……………………… ±40 VDC Measurement Category …………………….. CAT I

Power Requirements From SCXI Backplane
V+………………………………………………….. 18.5 to 25 VDC, 75 mA V­………………………………………………….. ­18.5 to ­25 VDC, 23 mA +5 V……………………………………………….. +4.75 to 5.25 VDC, 20.2 mA

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Appendix A Specifications

Environmental Physical

Operating temperature ……………………….0 to 50 °C Storage temperature …………………………..­20 to 70 °C Humidity ………………………………………….10 to 90% RH, noncondensing Maximum altitude……………………………..2,000 meters Pollution Degree (indoor use only) ……..2

3.0 cm (1.2 in.)

17.2 cm (6.8 in.)
18.8 cm (7.4 in.) Figure A-1. SCXI-1581 Dimensions
Weight …………………………………………….731 gm (25.8 oz)

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Appendix A Specifications

Safety

The SCXI-1581 is designed to meet the requirements of the following standards of safety for electrical equipment for measurement, control, and laboratory use:
· IEC 61010-1, EN 61010-1
· UL 61010-1
· CAN/CSA-C22.2 No. 61010-1

Note For UL and other safety certifications, refer to the product label or visit ni.com/certification, search by model number or product line, and click the appropriate link in the Certification column.

Electromagnetic Compatibility
Emissions ……………………………………….. EN 55011 Class A at 10 m FCC Part 15A above 1 GHz
Immunity………………………………………… EN 61326:1997 + A2:2001, Table 1
EMC/EMI……………………………………….. CE, C-Tick, and FCC Part 15 (Class A) Compliant

Note For EMC compliance, operate this device with shielded cabling.

CE Compliance

The SCXI-1581 meets the essential requirements of applicable European Directives, as amended for CE marking, as follows:
Low-Voltage Directive (safety) …………. 73/23/EEC
Electromagnetic Compatibility Directive (EMC) ……………………………… 89/336/EEC

Note Refer to the Declaration of Conformity (DoC) for this product for any additional regulatory compliance information. To obtain the DoC for this product, visit ni.com/certification, search by model number or product line, and click the appropriate link in the Certification column.

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B
Removing the SCXI-1581
This appendix explains how to remove the SCXI-1581 from MAX and an SCXI chassis or PXI/SCXI combination chassis.
Removing the SCXI-1581 from MAX
To remove a module from MAX, complete the following steps after launching MAX:

1. Expand Devices and Interfaces. 2. Click the + next to NI-DAQmx and/or Traditional NI-DAQ Devices
   to expand the list of installed chassis. 3. Click the + next to the appropriate chassis to expand the list of installed
   modules. 4. Right-click the module or chassis you want to delete and click Delete. 5. A confirmation window opens. Click Yes to continue deleting the
   module or chassis or No to cancel this action.
   Note Deleting the SCXI chassis deletes all modules in the chassis. All configuration information for these modules is also lost.
   The SCXI chassis and/or SCXI module(s) should now be removed from the list of installed devices in MAX.
   Removing the SCXI-1581 from a Chassis
   Consult the documentation for the chassis and accessories for additional instructions and precautions. To remove the SCXI-1581 module from a chassis, complete the following steps while referring to Figure B-1:
   Note Figure B-1 shows an SCXI chassis, but the same steps are applicable to a PXI/SCXI combination chassis.

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Appendix B Removing the SCXI-1581
1. Power off the chassis. Do not remove the SCXI-1581 module from a chassis that is powered on.
2. If the SCXI-1581 is the module cabled to the E/M Series DAQ device, disconnect the cable.
3. Remove any terminal block that connects to the SCXI-1581. 4. Rotate the thumbscrews that secure the SCXI-1581 to the chassis
counterclockwise until they are loose, but do not completely remove the thumbscrews.
Remove the SCXI-1581 by pulling steadily on both thumbscrews until the module slides completely out.

6

4

5
®
5 4 3 2 1
SCXI MAINFRAME

ADDRESS

1

SCXI

1100

3
1 Cable 2 SCXI Module Thumbscrews

2

3 SCXI-1581 4 Terminal Block

5 SCXI Chassis Power Switch 6 SCXI Chassis

Figure B-1. Removing the SCXI-1581

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Common Questions

This appendix lists common questions related to the use of the SCXI-1581.
Which version of NI-DAQ works with the SCXI-1581, and how do I get the most current version of NI-DAQ? You must have NI-DAQ 7.0 or later. Visit the NI Web site at ni.com and select Download Software»Drivers and Updates»Search Drivers and Updates. Enter the keyword NI-DAQ to find the latest version of NI-DAQ for your operating system.
Can the SCXI-1581 be the cabled module when there are also multiplexed analog- input modules in the chassis? Yes, unless one or more of the multiplexed analog-input modules in the same chassis feature simultaneous sampling (such as the SCXI-1140, SCXI-1520, SCXI-1530, or SCXI-1531). In that case, you must cable to a simultaneous-sampling module.
Can the SCXI-1581 current outputs be interactively controlled in MAX or programmatically controlled using NI-DAQ function calls, LabVIEW, or Measurement Studio? No. The current-output level is 100 µA as long as the chassis is powered on. You cannot power off or adjust the current output using MAX, NI-DAQ function calls, or an ADE such as LabVIEW or Measurement Studio. If you require this functionality, consider using an SCXI-1124 module or NI 670X device instead.
Can I create virtual channels for an SCXI-1581 current-output channel? No. As discussed above, you cannot programmatically control the current outputs.

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Appendix C Common Questions
Can I connect N current-output channels in parallel to create a precision current source that provides N × 100 µA? Yes, you can connect the current output in parallel. When connecting the output in parallel, connect the appropriate EX+ terminals together and the corresponding EX­ terminals together.
Can I connect N current-output channels in series to achieve a higher terminal-voltage compliance limit? No. Each current source is ground referenced. Therefore, you cannot place multiple current-outputs in series.
Are the SCXI-1581 current output channels isolated with respect to each other, the E/M Series DAQ device, or ground? No. The SCXI-1581 does not contain any isolation circuitry. All current outputs are referenced to chassis ground. If you require isolated current outputs, consider the SCXI-1124 module instead.
Can I modify the SCXI-1581 circuitry to generate current at a level different than 100 µA? No. Do not attempt to modify any circuitry in the SCXI-1581.
Are there any user-serviceable parts inside the SCXI-1581? No. There are no fuses, potentiometers, switches, socketed resistors, or jumpers inside the module. Disassembly of the module for any reason can void its warranty and nullify its accuracy specification.
Can I access the unused analog-input channels of the E/M Series DAQ device if it is directly cabled to the SCXI-1581 in a single-chassis system? Yes. E/M Series DAQ device channels 1 through 7 are available to measure unconditioned signals. Use an SCXI-1180 or the 50-pin breakout connector on the SCXI-1346 or SCXI-1349 cable adapter to route signals to these channels.

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Appendix C Common Questions

Which digital lines are unavailable on the E/M Series DAQ device if I am cabled to an SCXI-1581 module?
Table C-1 shows the digital lines that are used by the SCXI-1581 for communication and scanning. These lines are unavailable for general-purpose digital I/O if the SCXI-1581 is connected to the DAQ device.

Table C-1. SCXI-1581 Digital Signals

DAQ Signal Name

SCXI Signal Name

DIO0

SERDATIN

DIO4

SERDATOUT

DIO1

DAQD*/A

DIO2

SLOT0SEL*

SCANCLK

SCANCLK

1 With respect to the E/M Series DAQ device.

50-Pin Connector
25 26 27 29 36

68-Pin Connector
52 19 17 49 46

Direction1 Output Input Output Output Output

Does short-circuiting a current-output channel do any damage to the SCXI-1581? No. The SCXI-1581 delivers 100 µA into any load from 0 to 100 k.
Does open-circuiting a current-output channel damage the SCXI-1581? What is the open-circuit voltage level?
No. An SCXI-1581 current-output channel is not damaged if no load is connected. The open-circuit voltage is 12.4 VDC.
How can I tell if the SCXI-1581 is working?
Disconnect all loads from the channel you are testing. Measure the current- output value using an ammeter. If the ammeter has a fuse, ensure that the fuse is not blown before performing the measurement. Alternatively, place a known resistance R (<100 k) across the current output and use a voltmeter to measure the voltage drop across it. If the current output is functioning, you should measure
V = (100 × 10­6A) × R where R is given in .

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Appendix C Common Questions
How does the screw-terminal labeling on the SCXI-1300 relate to the front signal connector on the SCXI-1581? In other words, if I want to use current- output channel x, where do I connect wires in the SCXI-1300?
CH x + corresponds to EX x +. Likewise, CH x ­ corresponds to EX x ­. You cannot use any screw terminals in the SCXI-1300 other than CH x +, CH x ­, and chassis ground.
Can I use the BNC-2095 as an accessory for the SCXI-1581?
Yes. However, you must set all the slide switches on the back of the BNC-2095 for no bias-resistor connections to ground and no pull-up resistor connections to +5 V. In other words, set all switches on the rear of the BNC-2095 to the OFF (down) position.
When using an SCXI-1300 or BNC-2095 with the SCXI-1581, can I read the CJC sensor on these accessories?
No.
Can I use an SCXI-1303 terminal block as an accessory for the SCXI-1581?
NI does not recommend this combination. The SCXI-1303 is optimized for temperature measurement using thermocouples. The SCXI-1303 works with the SCXI-1581, but only if you remove all of the resistor packs. Refer to the SCXI-1303 32-Channel Isothermal Terminal Block Installation Guide for more information. As with the SCXI-1300 and BNC-2095, you cannot measure the CJC sensor.

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Glossary

Symbol µ m k M

Prefix micro milli kilo mega

Value 10 ­ 6 10­3 103 106

Numbers/Symbols

%

percent

positive of, or plus

­

negative of, or minus

±

plus or minus

<

less than

/

per

°

degree

ohms

+5 V (signal)

+5 VDC source signal

A
A ADE
AI AIGND

amperes
application development environment such as LabVIEW, LabWindows/CVI, Visual Basic, C, and C++
analog input analog input ground signal

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Glossary

B

bit

one binary digit, either 0 or 1

C

CE CGND CH

European emissions control standard chassis ground signal channel

channel chassis

pin or wire lead to which you apply, or from which you read, an analog or digital signal. Analog signals can be single-ended or differential. For digital signals, channels (also known as lines) are grouped to form ports.
the enclosure that houses, powers, and controls SCXI modules

CLK

clock input signal

common-mode voltage voltage that appears on both inputs of a differential amplifier

current excitation

a source that supplies the current needed by a sensor for its proper operation

D
D/A D/A DAQ
DAQ device DAQD/A

digital-to-analog
Data/Address
data acquisition–(1) collecting and measuring electrical signals from sensors, transducers, and test probes or fixtures and processing the measurement data using a computer; (2) collecting and measuring the same kinds of electrical signals with A/D and/or DIO devices plugged into a computer, and possibly generating control signals with D/A and/or DIO devices in the same computer
a data acquisition device. Examples are E/M Series data acquisition devices.
the data acquisition device data/address line signal used to indicate whether the SERDATIN pulse train transmitted to the SCXI chassis contains data or address information

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Glossary

device
DGND differential amplifier
DIN DIO DoC drivers/driver software

a plug-in data acquisition device, module, card, or pad that can contain multiple channels and conversion devices. SCXI modules are distinct from devices, with the exception of the SCXI-1200, which is a hybrid.
digital ground signal
an amplifier with two input terminals, neither of which are tied to a ground reference, whose voltage difference is amplified Deutsche Industrie Norme (German Industrial Standard)
digital input/output
Declaration of Conformity
software that controls a specific hardware device such as an E/M Series DAQ device

E
EMC EMI EX+ EX­ excitation EXTCLK

electromagnetic compliance electromagnetic interference positive excitation channel negative excitation channel a voltage or current source used to energize a sensor or circuit external clock signal

G

gain

the factor by which a signal is amplified, sometimes expressed in decibels

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Glossary
I
ID in. input impedance
J
jumper
L
lead resistance
M
m M
MISO MOSI multiplex multiplexed mode

identifier inch or inches the measured resistance and capacitance between the input terminals of a circuit
a small rectangular device used to connect two adjacent posts on a circuit board. Jumpers are used on some SCXI modules and terminal blocks to either select certain parameters or enable/disable circuit functionality.
the small resistance of a lead wire. The resistance varies with the lead length and ambient temperature. If the lead wire carries excitation current, this varying resistance can cause measurement error.
meters (1) Mega, the standard metric prefix for 1 million or 106, when used with units of measure such as volts and hertz; (2) mega, the prefix for 1,048,576, or 220, when used with B to quantify data or computer memory master-in-slave-out signal master-out-slave-in signal to route one of many input signals to a single output an SCXI operating mode in which analog input channels are multiplexed into one module output so that the cabled E/M Series DAQ device has access to the multiplexed output of the module as well as the outputs of all other multiplexed modules in the chassis

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N
NC NI-DAQ
NI-DAQmx
O
output voltage compliance OUTREF
P
ppm PXI
R
RL RMA RSVD RTD
S
s S

Glossary
not connected (signal) the driver software needed in order to use National Instruments E/M Series DAQ devices and SCXI components The latest NI-DAQ driver with new VIs, functions, and development tools for controlling measurement devices.
the largest voltage that can be generated across the output of a current source without the current going out of specification output reference signal
parts per million PCI eXtensions for Instrumentation–an open specification that builds on the CompactPCI specification by adding instrumentation-specific features
lead resistance Return Material Authorization reserved bit, pin, or signal resistance-temperature detector
seconds samples

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Glossary

scan
SCANCLK SCXI SCXIbus sensor SERCLK SERDATIN SERDATOUT signal conditioning Slot 0 SLOT0SEL SPICLK
T
thermistor Traditional NI-DAQ (Legacy)
transducer

one or more analog samples taken at the same time, or nearly the same time. Typically, the number of input samples in a scan is equal to the number of channels in the input group. For example, one scan, acquires one new sample from every analog input channel in the group. scan clock signal used to increment to the next channel after each E/M Series DAQ device analog-to- digital conversion
Signal Conditioning eXtensions for Instrumentation
located in the rear of an SCXI chassis, the SCXIbus is the backplane that connects modules in the same chassis to each other
a type of transducer that converts a physical phenomenon into an electrical signal
serial clock signal used to synchronize digital data transfers over the SERDATIN and SERDATOUT lines serial data input signal
serial data output signal
the manipulation of signals to prepare them for digitizing
refers to the power supply and control circuitry in the SCXI chassis
slot 0 select signal
serial peripheral interface clock signal
a thermally sensitive resistor
An upgrade to the earlier version of NI-DAQ. Traditional NI-DAQ (Legacy) has the same VIs and functions and works the same way as NI-DAQ 6.9.x. You can use both Traditional NI-DAQ (Legacy) and NI-DAQmx on the same computer, which is not possible with NI-DAQ 6.9.x.
a device capable of converting energy from one form to another

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U
UL
V
V VAC VDC VI
virtual channels
W
W

Glossary
Underwriters Laboratory
volts volts, alternating current volts, direct current virtual instrument–(1) a combination of hardware and/or software elements, typically used with a PC, that has the functionality of a classic stand-alone instrument; (2) a LabVIEW software module (VI), which consists of a front panel user interface and a block diagram program channel names that can be defined outside the application and used without having to perform scaling operations
watts

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Index

Numerics
2-wire configuration of resistive devices, 4-3 3-wire resistive sensor
connected in 2-wire configuration, 4-4 lead-resistance compensation
with two differential amplifiers, 4-6 with two matched current sources, 4-5 4-wire configuration of resistive devices, 4-2
B
BNC-2905 rack-mount accessory, questions about, C-4

D
DAQ device accessing unused analog input channels, C-2 digital signals unavailable with DAQ device connected (table), C-3
E
electromagnetic compatibility specifications, A-3
environment specifications, A-2 excitation specifications, A-1

C
CE compliance specifications, A-3 common questions, C-1 connecting resistive devices to SCXI-1581
2-wire configuration, 4-3 3-wire resistive sensor connected in 2-wire
configuration, 4-4 4-wire configuration, 4-2 lead-resistance compensation
using 3-wire resistive sensor and two differential amplifiers, 4-6 and two matched current sources, 4-5
connectors, rear signal connector communication signals, 2-3
conventions used in the manual, viii current output channels, questions
about, C-1, C-3 current sources, operating, 4-1

F
front connector, pin assignments (table), 2-2
I
installation, into SCXI chassis, 1-4
M
Measurement & Automation Explorer, removing the SCXI-1581, B-1
multiplexed scanning, scanning other SCXI modules through SCXI-1581, 3-2
N
NI-DAQ version required, C-1

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O
operation of current sources, 4-1
P
physical specifications, A-2 pin assignments, front connector (table), 2-2 power requirements from SCXI
backplane, A-1
Q
questions and answers, C-1
R
rear signal connector, communications signals, 2-3
removing the SCXI-1581 from Measurement & Automation Explorer, B-1
resistive devices, connecting to SCXI-1581 2-wire configuration, 4-3 3-wire resistive sensor connected to 2-wire configuration, 4-4 4-wire configuration, 4-2 lead-resistance compensation using 3-wire resistive sensor and two differential amplifiers, 4-6 using 3-wire resistive sensor and two matched current sources, 4-5
RTDs (resistive-temperature detectors) measurement errors, 4-9 overview, 4-9 relationship between resistance and temperature, 4-10 resistance-temperature curve (figure), 4-11

S
safety specifications, A-3 SCXI-1303 terminal block, C-4 SCXI-1581
common questions, C-1 digital signals unavailable signals with
DAQ device connected (table), C-3 removing (figure), B-2 removing from SCXI chassis, B-1 taking measurements. See measurements signal connections, front connector pin assignments (table), 2-2 software, NI-DAQ version required, C-1 specifications CE compliance, A-3 electromagnetic compatibility, A-3 environment, A-2 excitation, A-1 physical, A-2 power requirements from SCXI
backplane, A-1 safety, A-3 stability, A-1 stability specifications, A-1

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T
taking measurements. See measurements temperature measurement with resistive
transducers, 4-9 connecting resistive devices to SCXI-1581, 4-1 2-wire configuration, 4-3 3-wire resistive sensor connected in 2-wire configuration, 4-4 4-wire configuration, 4-2 lead resistance compensation using 3-wire resistive sensor and two differential amplifiers, 4-6 using 3-wire resistive sensor and two matched current sources, 4-5

Index
RTDs measurement errors, 4-9 overview, 4-9 relationship between resistance and temperature, 4-10 resistance temperature curve (figure), 4-11
thermistors measurement circuits, 4-15 overview, 4-13 resistance/temperature characteristics, 4-16 resistance-temperature curve (figure), 4-14

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References

• Test and Measurement Systems, a part of Emerson - NI
• Product Certifications - NI
• Using Info Codes - NI
• Product Documentation - NI

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