Month: July 2025

General Description

This module mounts into 1746 I/O chassis for use with SLC 500 fixed

and modular systems. The module stores digitally converted

thermocouple/mV analog data in its image table for retrieval by all

fixed and modular SLC 500 processors. The module supports

connections from any combination of up to eight thermocouple/mV analog sensors.

Input Ranges

The following tables define thermocouple types and associated

temperature ranges and the millivolt analog input signal ranges that

each of the module’s input channels support. To determine the

practical temperature range of your thermocouple, refer to the

specifications in Appendix A.

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.

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.

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.

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.

Module Operation

Each input channel consists of an RTD connection, which provides:

· excitation current

· a sense connection, which detects lead-wire resistance

· a return connection, which reads the RTD or resistance value

Each of these analog inputs are multiplexed to an analog converter.

The A/D converter cycles between reading the RTD or resistance value, the

lead wire resistance, and the excitation current. From these readings, an

accurate temperature or resistance is returned to the user program.

The RTD module is isolated from the chassis backplane and chassis ground.

The isolation is limited to 500V ac. Optocouplers are used to communicate

across the isolation barrier. Channel-to-channel common-mode isolation is

limited to ± 5 volts.

LED Status

The illustration below shows the RTD module LED panel consisting of nine

LEDs. The state of the LEDs (for example, off, on, or flashing) depends on the

operational state of the module (see table on page 1-9).

Power-up

At power-up, the RTD module checks its internal circuits, memory, and basic

functions via hardware and software diagnostics. During this time, the module

status LED remains off, and the channel status LEDs are turned on. If no

faults are found during the power-up diagnostics, the module status LED is

turned on, and the channel status LEDs are turned off.

After power-up checks are complete, the RTD 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 bits are set by the user program, the channel

status LEDs go on and the module continuously converts the RTD or

resistance input to a value within the range you selected for the enabled

channels. The module is now operating in its normal state.

Each time a channel is read by the module, that data value is tested by the

module for a fault condition, for example, open-circuit, short-circuit, overrange,

and under range. If such a condition is detected, a unique bit is set in

the channel status word and the channel status LED flashes, indicating a

channel error condition.

The SLC processor reads the converted RTD or resistance data from the

module at the end of the program scan or when commanded by the ladder

program. The processor and RTD module determine that the backplane data

transfer was made without error and the data is used in your ladder program.

System Overview

Each individual channel on the RTD module can receive input signals from 2.

3 or 4-wire RTD sensors or from resistance input devices. You configure each

channel to accept either input. When configured for RTD input types, the

module converts the RTD readings into linearized, digital temperature

readings in °C or °F. When configured for resistance inputs, the module

provides a linear resistance value in ohms.

IMPORTANT 

The RTD module is designed to accept input from RTD

sensors with up to 3 wires. When using 4-wire RTD

sensors, one of the 2 lead compensation wires is not used

and the 4-wire sensor is treated like a 3-wire sensor. Lead

wire compensation is provided via the third wire. Refer

to Wiring Considerations on page 2-8 for more information.

System Operation

The RTD module has 3 operational states:

· power-up

· module operation

· error (module error and channel error)

General Diagnostic Features

The RTD module contains diagnostic features that can be used to 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 6. Module Diagnostics and Troubleshooting.

The RTD 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 RTD modules in your system as the power

supply can support, as shown in the illustration below.

System Overview

Each individual channel on the RTD module can receive input signals from 2.

3 or 4-wire RTD sensors or from resistance input devices. You configure each

channel to accept either input. When configured for RTD input types, the

module converts the RTD readings into linearized, digital temperature

readings in °C or °F. When configured for resistance inputs, the module

provides a linear resistance value in ohms.

Description

The RTD module supplies a small current to each RTD connected to the

module inputs (up to 8 input channels). The module provides on-board

scaling and converts RTD input to temperature (°C, °F) or reports resistance

input in ohms.

Each input channel is individually configurable for a specific input device.

Broken sensor detection (open- or short-circuit) is provided for each input

channel. In addition, the module provides indication if the input signal is

out-of-range. For more detail on module functionality, refer to the subsection

entitled System Overview later in this chapter.

General Diagnostic Features

The RTD module contains diagnostic features that can be used to 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 6. Module Diagnostics and Troubleshooting.

The RTD 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 RTD modules in your system as the power

supply can support, as shown in the illustration below.

Overview

This chapter describes the 8-channel 1746-NR8 RTD/Resistance Input

Module and explains how the SLC controller gathers RTD (Resistance

Temperature Detector) temperature or resistance-initiated analog input from

the module. Included is:

· a general description of the module’s hardware and software features

· an overview of system operation

For the rest of the manual, the 1746-NR8 RTD/Resistance Input Module is

referred to as simply the RTD module.

The RTD module receives and stores digitally converted analog data from

RTDs or other resistance inputs such as potentiometers into its image table for

retrieval by all fixed and modular SLC 500 processors. An RTD consists of a

temperature-sensing element connected by 2. 3. or 4 wires that provide input

to the RTD module. The module supports connections from any combination

of up to eight RTDs of various types (for example: platinum, nickel, copper, or

nickel-iron) or other resistance inputs.