A-B 1746-NT4 System Overview SLC™ 500 4-Channel Thermocouple/mV Input Module

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.

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.

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.