Allen-Bradley SLC 500 User manual

SLC ™ 500
4-Channel
Thermocouple/mV
Input Module
(Catalog Number 1746-NT4,
Series B)
User Manual

Important User Information
Solid state equipment has operational characteristics differing from those of
electromechanical equipment. Safety Guidelines for the Application,
Installation and Maintenance of Solid State Controls (Publication SGI-1.1
available from your local Rockwell Automation sales office or online at
http://www.ab.com/manuals/gi) describes some important differences
between solid state equipment and hard-wired electromechanical devices.
Because of this difference, and also because of the wide variety of uses for
solid state equipment, all persons responsible for applying this equipment
must satisfy themselves that each intended application of this equipment is
acceptable.
In no event will Rockwell Automation, Inc. be responsible or liable for
indirect or consequential damages resulting from the use or application of
this equipment.
The examples and diagrams in this manual are included solely for illustrative
purposes. Because of the many variables and requirements associated with
any particular installation, Rockwell Automation, Inc. cannot assume
responsibility or liability for actual use based on the examples and diagrams.
No patent liability is assumed by Rockwell Automation, Inc. with respect to
use of information, circuits, equipment, or software described in this manual.
Reproduction of the contents of this manual, in whole or in part, without
written permission of Rockwell Automation, Inc. is prohibited.
Throughout this manual we use notes to make you aware of safety
considerations.
WARNING Identifies information about practices or circumstances
that can cause an explosion in a hazardous environment,
which may lead to personal injury or death, property
damage, or economic loss.
IMPORTANT Identifies information that is critical for successful
application and understanding of the product.
ATTENTION Identifies information about practices or circumstances
that can lead to personal injury or death, property
damage, or economic loss. Attentions help you:
•identify a hazard
•avoid a hazard
•recognize the consequence
SHOCK HAZARD Labels may be located on or inside the drive to alert
people that dangerous voltage may be present.
BURN HAZARD Labels may be located on or inside the drive to alert
people that surfaces may be dangerous temperatures.

Publication 1746-UM007C-EN-P - July 2004
Summary of Changes
The information below summarizes the changes to this manual since
the last printing.
To help you find new and updated information in this release of the
manual, we have included change bars as shown to the right of this
paragraph.
For information on: See page:
Changes to the SLC™ 500 Thermocouple/mV Input Module. throughout manual
Using RSLogix™ 500 to configure the NT4 module. 2-4, 5-2, 6-1, and Appendix
E
Maintaining the ambient temperature surrounding the SLC
500 above 3°C (37.4°F). A-2

Publication 1746-UM007C-EN-P - July 2004
2 Summary of Changes

iPublication 1746-UM007C-EN-P - July 2004
Table of Contents
Preface
Who Should Use this Manual. . . . . . . . . . . . . . . . . . . . . . . P-1
Purpose of this Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . P-1
Related Documentation . . . . . . . . . . . . . . . . . . . . . . . . P-2
Your Questions or Comments on this Manual . . . . . . . . P-3
Common Techniques Used in this Manual . . . . . . . . . . . . . P-3
Chapter 1
Overview
General Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1
Hardware Features. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2
General Diagnostic Features. . . . . . . . . . . . . . . . . . . . . 1-3
System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3
System Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4
Module Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-5
Thermocouple Compatibility . . . . . . . . . . . . . . . . . . . . 1-5
Linear Millivolt Device Compatibility. . . . . . . . . . . . . . . 1-7
Chapter 2
Quick Start for Experienced Users
Required Tools and Equipment . . . . . . . . . . . . . . . . . . . . . 2-1
Installation Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2
Chapter 3
Installation and Wiring
Compliance to European Union Directives . . . . . . . . . . . . . 3-1
EMC Directive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1
Electrostatic Discharge. . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2
NT4 Power Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3-2
Module Location in Chassis . . . . . . . . . . . . . . . . . . . . . . . . 3-3
Fixed Expansion Chassis Considerations . . . . . . . . . . . . 3-3
General Considerations . . . . . . . . . . . . . . . . . . . . . . . . 3-5
Module Installation and Removal. . . . . . . . . . . . . . . . . . . . 3-5
Terminal Block Removal . . . . . . . . . . . . . . . . . . . . . . . 3-6
Module Installation Procedure . . . . . . . . . . . . . . . . . . . 3-6
Module Removal Procedure . . . . . . . . . . . . . . . . . . . . . 3-7
Terminal Wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7
Wiring Considerations . . . . . . . . . . . . . . . . . . . . . . . . . 3-8
Wiring Input Devices to the NT4 . . . . . . . . . . . . . . . . . 3-11
Cold Junction Compensation (CJC) . . . . . . . . . . . . . . . . 3-12
Thermocouple Calibration . . . . . . . . . . . . . . . . . . . . . . . . . 3-13
Chapter 4
Preliminary Operating
Considerations
Module ID Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1
Module Addressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2
Output Image-Configuration Words . . . . . . . . . . . . . . . 4-2
Input Image-Data Words and Status Words . . . . . . . . . . 4-3
Channel Filter Frequency Selection . . . . . . . . . . . . . . . . . . 4-4

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Table of Contents ii
Effective Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-4
Channel Cut-Off Frequency . . . . . . . . . . . . . . . . . . . . . 4-5
Channel Step Response . . . . . . . . . . . . . . . . . . . . . . . . 4-6
Update Time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7
Update Time Calculation Example . . . . . . . . . . . . . . . . 4-8
Channel Turn-On, Turn-Off, and Reconfiguration Times . . . 4-9
Response to Slot Disabling . . . . . . . . . . . . . . . . . . . . . . . . 4-10
Input Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-10
Output Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-10
Chapter 5
Channel Configuration, Data, and
Status
Channel Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1
Channel Configuration Procedure . . . . . . . . . . . . . . . . . . . 5-2
Select Input Type (Bits 0-3) . . . . . . . . . . . . . . . . . . . . . 5-5
Select Data Format (Bits 4 and 5) . . . . . . . . . . . . . . . . . 5-5
Using Scaled-for-PID and Proportional Counts . . . . . . . 5-6
Scaling Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-7
Select Open Circuit State (Bits 6 and 7). . . . . . . . . . . . . 5-10
Select Temperature Units (Bit 8) . . . . . . . . . . . . . . . . . . 5-11
Select Channel Filter Frequency (Bits 9 and 10). . . . . . . 5-11
Select Channel Enable (Bit 11) . . . . . . . . . . . . . . . . . . . 5-12
Unused Bits (Bits 12-15). . . . . . . . . . . . . . . . . . . . . . . . 5-12
Channel Data Word. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-12
Channel Status Checking . . . . . . . . . . . . . . . . . . . . . . . . . . 5-13
Status Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-15
Input Type Status (Bits 0-3) . . . . . . . . . . . . . . . . . . . . . 5-15
Data Format Type Status (Bits 4 and 5). . . . . . . . . . . . . 5-15
Open Circuit Type Status (Bits 6 and 7) . . . . . . . . . . . . 5-15
Temperature Units Type Status (Bit 8). . . . . . . . . . . . . . 5-16
Channel Filter Frequency (Bits 9 and 10) . . . . . . . . . . . 5-16
Channel Status (Bit 11). . . . . . . . . . . . . . . . . . . . . . . . . 5-16
Open-Circuit Error (Bit 12) . . . . . . . . . . . . . . . . . . . . . . 5-16
Under-Range Error (Bit 13). . . . . . . . . . . . . . . . . . . . . . 5-16
Over-Range Error (Bit 14). . . . . . . . . . . . . . . . . . . . . . . 5-17
Configuration Error (Bit 15) . . . . . . . . . . . . . . . . . . . . . 5-17
Chapter 6
Ladder Programming Examples
Initial Programming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1
Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2
Dynamic Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3
Verifying Channel Configuration Changes . . . . . . . . . . . . . 6-4
Interfacing to the PID Instruction. . . . . . . . . . . . . . . . . . . . 6-5
Monitoring Channel Status Bits . . . . . . . . . . . . . . . . . . . . . 6-6
Invoking Autocalibration . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7

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Table of Contents iii
Chapter 7
Module Diagnostics and
Troubleshooting
Module Operation vs Channel Operation . . . . . . . . . . . . . . 7-1
Power-up Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1
Channel Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2
LED Indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3
Channel Status LEDs (Green) . . . . . . . . . . . . . . . . . . . . 7-4
Invalid Channel Configuration . . . . . . . . . . . . . . . . . . . 7-4
Open Circuit Detection . . . . . . . . . . . . . . . . . . . . . . . . 7-4
Out-Of-Range Detection. . . . . . . . . . . . . . . . . . . . . . . . 7-5
Module Status LED (Green) . . . . . . . . . . . . . . . . . . . . . 7-5
Troubleshooting Flowchart . . . . . . . . . . . . . . . . . . . . . . . . 7-6
Replacement Parts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-7
Contacting Rockwell Automation . . . . . . . . . . . . . . . . . . . . 7-7
Chapter 8
Application Examples
Basic Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1
Application Setup (Display a Temperature) . . . . . . . . . 8-1
Channel Configuration . . . . . . . . . . . . . . . . . . . . . . . . . 8-2
Supplementary Example . . . . . . . . . . . . . . . . . . . . . . . . . . 8-4
Application Setup (Four Channels °C - °F) . . . . . . . . . . 8-4
Device Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . 8-4
Channel Configuration . . . . . . . . . . . . . . . . . . . . . . . . . 8-5
Program Setup and Operation Summary . . . . . . . . . . . . 8-7
Program Listing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-7
Appendix A
Specifications
Electrical Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . A-1
Physical Specifications. . . . . . . . . . . . . . . . . . . . . . . . . . . . A-2
Environmental Specifications . . . . . . . . . . . . . . . . . . . . . . . A-2
Input Specifications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-3
1746-NT4 Module Accuracy . . . . . . . . . . . . . . . . . . . . . A-4
Input Resolution per Thermocouple Type at Each Filter
Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A-4
Appendix B
NT4 Configuration Worksheet
Channel Configuration Procedure . . . . . . . . . . . . . . . . . . . B-1
Channel Configuration Worksheet . . . . . . . . . . . . . . . . . . . B-4
Appendix C
Thermocouple Restrictions
J Type Thermocouple . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1
(Iron vs. Copper-Nickel <Constantan>) . . . . . . . . . . . . . C-1
K Type Thermocouple . . . . . . . . . . . . . . . . . . . . . . . . . . . C-2
(NIckel-Chromium vs. Nickel-Aluminum) . . . . . . . . . . . C-2
T Type Thermocouple. . . . . . . . . . . . . . . . . . . . . . . . . . . . C-3

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Table of Contents iv
(Copper vs. Copper-Nickel <Constantan>) . . . . . . . . . . C-3
E Type Thermocouple. . . . . . . . . . . . . . . . . . . . . . . . . . . . C-4
(Nickel-Chromium vs. Copper-Nickel <Constantan>) . . . C-4
S and R Type Thermocouples . . . . . . . . . . . . . . . . . . . . . . C-5
S (Platinum-10% Rhodium vs. Platinum)
R (Platinum-13% Rhodium vs. Platinum) . . . . . . . . . . . C-5
Appendix D
Thermocouple Types
Appendix E
Configuring the 1746-NT4 Module
with RSLogix 500
Glossary
Index

1Publication 1746-UM007C-EN-P - July 2004
Preface
Read this preface to familiarize yourself with the rest of the manual.
The preface includes:
•Who Should Use this Manual
•Purpose of this Manual
•Common Techniques Used in this Manual
Who Should Use this
Manual
Use this manual if you are responsible for designing, installing,
programming, or troubleshooting control systems that use SLC 500
4-Channel Thermocouple/mV Input Module.
You should have a basic understanding of electrical circuitry and
familiarity with relay logic. If you do not, obtain the proper training
before using this product.
Purpose of this Manual
This manual describes the procedures you use to install, wire, and
troubleshoot your 4-channel thermocouple/mV module. This manual:
•explains how to install and wire your module
•gives you an overview of the SLC 500 programmable controller
system
Refer to your programming software user documentation for more
information on programming your SLC 500 programmable controller.

Publication 1746-UM007C-EN-P - July 2004
2 Preface
Related Documentation
The following documents contain additional information concerning
Rockwell Automation products. To obtain a copy, contact your local
Rockwell Automation office or distributor.
For Read this Document Document Number
In-depth information on the SLC Instruction Set. SLC 500 Instruction Set Reference Manual 1747-RM001
A description on how to install and use your Modular SLC 500
programmable controller. SLC 500 Modular Hardware Style User Manual 1747-UM011
A description on how to install and use your Fixed SLC 500
programmable controller. Installation & Operation Manual for Fixed Hardware Style Programmable Controllers 1747-6.21
A description on how to install the SLC 500 4-Channel
Thermocouple/mV input module SLC 500 4-Channel Thermocouple/mV Module Installation Instructions 1746-IN010
Information on reducing electrical noise. System Design for Control of Electrical
Noise GMC-RM001
In-depth information on grounding and wiring Allen-Bradley®
programmable controllers. Allen-Bradley Programmable Controller Grounding and Wiring Guidelines 1770-4.1
A description of important differences between solid-state
programmable controller products and hard-wired electromechanical
devices.
Application Considerations for Solid-State
Controls SGI-1.1
An article on wire sizes and types for grounding electrical
equipment. National Electrical Code - Published by the National Fire Protection Association of Boston, MA.
A glossary of industrial automation terms and abbreviations. Allen-Bradley Industrial Automation
Glossary AG-7.1

Publication 1746-UM007C-EN-P - July 2004
Preface 3
If you would like a manual, you can:
•download an electronic version from the internet at:
–www.theautomationbookstore.com
–http://www.ab.com/manuals
•order a printed manual by:
–contacting your local distributor or Rockwell Automation
representative
–visiting www.theautomationbookstore.com
–calling 1.800.963.9548 (USA/Canada) or 001.330.725.1574
(Outside USA/Canada)
Your Questions or Comments on this Manual
If you find a problem with this manual, or you have any suggestions
for how this manual could be made more useful to you, please
contact us at the address below:
Rockwell Automation
Automation Control and Information Group
Technical Communication, Dept. A602V
P.O. Box 2086
Milwaukee, WI 53201-2086
Common Techniques Used
in this Manual
The following conventions are used throughout this manual:
•Bulleted lists such as this one provide information, not
procedural steps.
•Numbered lists provide sequential steps or hierarchical
information.
•Italic type is used for emphasis.

Publication 1746-UM007C-EN-P - July 2004
4 Preface

1Publication 1746-UM007C-EN-P - July 2004
Chapter 1
Overview
This chapter describes the thermocouple/millivolt module and
explains how the SLC controller gathers thermocouple or millivolt
initiated analog input from the module. This chapter includes:
•General Description
•System Overview
General Description
The thermocouple/mV module receives and stores digitally converted
thermocouple and/or millivolt (mV) analog data into its image table
for retrieval by all fixed and modular SLC 500 processors. The module
supports connections from any combination of up to four
thermocouple or mV analog sensors.
The following tables define thermocouple types and their associated
full scale temperature ranges and also list the millivolt analog input
signal ranges that each 1746-NT4 channel will support. To determine
the practical temperature range your thermocouple supports, refer to
the specifications in Appendix A.
Each input channel is individually configurable for a specific input
device and provides open-circuit, over-range, and under-range
detection and indication.
Type °C Temperature Range °F Temperature Range
J -210° to 760° -346° to 1400°
K -270° to 1370° -454° to 2498°
T -270° to 400° -454° to 752°
B 300° to 1820° 572° to 3308°
E -270° to 1000° -454° to 1832°
R 0° to 1768° 32° to 3214°
S 0° to 1768° 32° to 3214°
N 0° to 1300° 32° to 2372°
CJC Sensor 0° to 85° 32° to 185°
Millivolt Input Type Range
±50 mV -50 mV dc to +50 mV dc
±100 mV -100 mV dc to +100 mV dc

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1-2 Overview
Hardware Features
The thermocouple module fits into any single-slot, except the
processor slot (0), in either an SLC 500 modular system or an SLC 500
fixed system expansion chassis (1746-A2). It is a Class 1 module (uses
8 input words and 8 output words). It interfaces to thermocouple
types J, K, T, E, R, S, B, and N, and supports direct ±50 mV and ±100
mV analog input signals.
The module requires the use of Block Transfer in a remote
configuration.
The module contains a removable terminal block providing
connection for four thermocouple and/or analog input devices. There
are also two, cold-junction compensation (CJC) sensors used to
compensate for offset voltages introduced into the input signal as a
result of the cold-junction, i.e., where the thermocouple wires connect
to the module wiring terminal. There are no output channels on the
module. Module configuration is done via the user program. There are
no DIP switches.
Cable Tie Slots
SLC 500
CAT SERIAL
NO.
INPUT
Self-Locking Tabs
THERMOCOUPLE/mV INPUT MODULE
MADE IN USA
Side Label
Removable
Terminal Block
FAC 1M
INPUT SIGNAL RANGES
THERMOCOUPLE TYPES: VOLTAGE:
100mVDC to +100mVDC
CJC A+
SHIELD
SHIELD
SHIELD
SHIELD
CHL3
CHL3+
CHL2
CHL2+
CHL1
CHL1+
CHL0
CHL0+
Door Label
CJC Sensors
Do Not
Remove
CJC A
Do Not
Remove
CJC B
Do Not
Remove
CJC B+
Do Not
Remove
SHIELD
ANLG
50mVDC to +50mVDC
SER FRN
ULLISTED IND. CONT. EQ.
FOR HAZ. LOC. A196
CLASS I, GROUPS A, B, C AND D, DIV.2
OPERATING
SA J, K, T, E, R, S, B, N
TEMPERATURE CODE T3C
MODULE ST ATUS
0
1
2
3
CHANNEL
STATUS
THERMOCOUPLE/mV
1746 NT4 NT4-xxx x
Channel Status
LEDs (Green)
Module Status
LED (Green)
COM
_
_
_
_
_
_
®®
_
_

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Overview 1-3
General Diagnostic Features
The thermocouple/mV module contains diagnostic features that can
help you identify the source of problems that may occur during
power-up or during normal channel operation. These power-up and
channel diagnostics are explained in chapter 7, Module Diagnostics
and Troubleshooting.
System Overview
The thermocouple module communicates to the SLC 500 processor
through the parallel backplane interface and receives +5V dc and
+24V dc power from the SLC 500 power supply through the
backplane. No external power supply is required. You may install as
many thermocouple modules in your system as the power supply can
support.
Hardware Function
Channel Status LED
Indicators Display operating and fault status of channels 0, 1, 2, and 3
Module Status LED Displays module operating and fault status
Side Label (Nameplate) Provides module information
Removable Terminal Block Provides physical connection to input devices.
It is color coded green.
Door Label Permits easy terminal identification
Cable Tie Slots Secure and route wiring from module
Self-Locking Tabs Secure module in chassis slot
Thermocouple Modules
SLC Processor

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1-4 Overview
Each individual channel on the thermocouple module can receive
input signals from thermocouple sensors or mV analog input devices.
You configure each channel to accept either input. When configured
for thermocouple input types, the thermocouple module converts the
analog input voltages into cold-junction compensated and linearized,
digital temperature readings. The 1746-NT4 uses the National Bureau
of Standards (NBS) Monograph 125 and 161 based on IPTS-68 for
thermocouple linearization.
When configured for millivolt analog inputs, the module converts the
analog values directly into digital values. The module assumes that the
mV input signal is already linear.
System Operation
At power-up, the thermocouple module performs a check of its
internal circuits, memory, and basic functions. During this time the
module status LED remains off. If no faults are found during the
power-up diagnostics, the module status LED is turned on.
After power-up checks are complete, the thermocouple module waits
for valid channel configuration data from your SLC ladder logic
program (channel status LEDs off). After configuration data is written
to one or more channel configuration words and their channel enable
status bits are set, the channel status LEDs go on and the
thermocouple module continuously converts the thermocouple or
millivolt input to a value within the range you selected for the enabled
channels.
Each time a channel is read by the module, that data value is tested by
the module for a fault condition, i.e. open circuit, over range, and
under range. If such a condition is detected, a unique bit is set in the
channel status word and the channel status LED blinks.
The SLC processor reads the converted thermocouple or millivolt data
from the module at the end of the program scan, or when
commanded by the ladder program. The processor and thermocouple
module determine that the backplane data transfer was made without
error, and the data is used in your ladder program.
Thermocouple
Input
Module
SLC 500
Processor
Channel Configuration W ord
Channel Data W ord
Thermocouple or mV
Analog Signals
Channel Status W ord

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Overview 1-5
Module Operation
The thermocouple module input circuitry consists of four differential
analog inputs multiplexed into a single analog-to-digital (A/D)
convertor. The mux circuitry also continuously samples the CJC A and
CJC B sensors and compensates for temperature changes at the cold
junction (terminal block). The figure on the following page shows a
block diagram for the analog input circuitry.
The A/D convertor reads the selected input signal and converts it to a
digital value. The multiplexer sequentially switches each input
channel to the module’s A/D convertor. Multiplexing provides an
economical means for a single A/D convertor to convert multiple
analog signals. However, it does affect the speed at which an input
signal can change and still be detected by the convertor.
Thermocouple Compatibility
The thermocouple module is fully compatible with all SLC 500 fixed
and modular controllers. It is compatible with all NBS MN-125
standard types J, K, T, E, R, S, and B thermocouple sensors and
extension wire; and with NBS MN-161, 14AWG, standard type N
thermocouple and extension wire. Refer to Appendix C for more
details.
The Series B (or higher) 1746-NT4 differential design allows for a
maximum channel-to-channel common-mode voltage
difference/separation of 2 volts. This means that if you are using an
NT4 with multiple grounded thermocouples with metallic sheaths or
exposed thermocouples with measuring junctions that make contact
with electrically conductive material, their ground potentials must be
within 2 volts. If this is not done, your temperature readings will be
inaccurate or the module could be damaged. If your grounded
thermocouple protective sheath is made of an electrically
non-conductive material such as ceramic, then the voltage separation
specification is not as important. Refer to Appendix D for an
explanation of grounded, ungrounded, and exposed thermocouples.
Use the analog common (ANALOG COM) terminal for applications that
have multiple grounded thermocouples. This analog common
terminal must be jumpered to either the (+) or (-) terminal of any
active channel which is connected to a grounded thermocouple. See
Wiring Considerations on page 3-8 for complete information on the
use of the ANALOG COM terminal.

Publication 1746-UM007C-EN-P - July 2004
1-6 Overview
Input Circuit Block Diagram
Input Circuit Block Diagram
Digital
Value
User-Selected
Filter Frequency
Digital
Filter
Analog to
Digital
Convertor
Multiplexer
Open Circuit
Detection
+
-
Shield
+
-
+
-
Shield
+
-
Shield
+
-
+
-
CJCA Sensor
CJCB Sensor
TerminalBlock Module Circuitry
Chassis Ground
(internally connected)
Channel 1
+
-
Shield
user supplied
jumper
grounded
thermocouple
Channel 2
within
2V*
grounded
thermocouple
Channel 3
Channel 0
within
2V*
grounded
thermocouple
ungrounded
thermocouple
Analog
Common
*See Important note below.
IMPORTANT When using multiple grounded and/or exposed
thermocouples that are touching on electrically
conductive material with Series B or higher
1746-NT4, the ground potential between any two
channels cannot exceed 2 volts.

Publication 1746-UM007C-EN-P - July 2004
Overview 1-7
Linear Millivolt Device Compatibility
A large number of millivolt devices may be used with the 1746-NT4
module. For this reason we do not specify compatibility with any
particular device.
However, millivolt applications often use bridges of strain gages. To
allow the NT4 Series B (or higher) to operate correctly, the analog
common (ANALOG COM) terminal of the module needs to be biased to a
level within 2V of the signal of interest. A resistive voltage divider
using 10k Ωresistors is recommended to accomplish this. The circuit
diagram below shows how this connection is made.
ATTENTION The possibility exists that grounded or exposed
thermocouples can become shorted to a potential
greater than that of the thermocouple itself. Due to
possible shock hazard, care should be taken when
wiring these types of thermocouples. Refer to
Appendix D for more details.
+
-
ANALOG COM
fixed
fixed
variable
fixed
INPUT
(CHL0, CHL1,
CHL2, CHL3)
Vcc +
10k Ω
10k Ω
NT4
Strain
Gage
Bridge

Publication 1746-UM007C-EN-P - July 2004
1-8 Overview