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

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.