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.