Rockwell Automation 1746-INT4 Thermocouple/MV Isolated/ User Manual - Manual (Page 13)

Publication 1746-UM614B-EN-P - September 2007
Module Overview 13
System Operation
When you cycle power, the module checks its internal circuits,
memory, and basic functions. During this time, the module status
indicator remains off. If the module finds no faults, it turns on its
module status indicator.
System Operation
After completing power cycle checks, the module waits for valid
channel configuration data from your SLC ladder logic program
(channel status LED indicators are off). After channel configuration
data is transferred and channel enable bits are set for one or more
channels, the module turns on its channel status LED indicators. Then
it continuously converts the thermocouple or millivolt input to a value
within the range you selected for the channel.
Each time the module reads an input channel, the module tests that
data for a fault, for example, open-circuit, over-range, or under-range
condition. If it detects such a condition, the module sets a unique bit
in the channel status word and causes the channel status indicator to
blink.
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. After the processor and module
determine that the data transfer was made without error, the data can
be used in your ladder program.
Module Operation
The module’s input circuitry consists of four differential analog inputs,
each with its own analog-to-digital (A/D) convertor. The A/D
convertors read the analog input signals and convert them to digital
counts. The input circuitry also continuously samples the CJC sensors
and compensates for temperature changes at the cold-junction
(terminal block).
See page 15 for a block diagram for the analog input circuitry.
Channel Data Word
Channel Status Word
Channel Configuration Word
SLC 500
Processor Thermocouple
Input Module
Thermocouple
or mV Analog
Signals