Rockwell Automation 1746-NT4 Series B,D17466.6.1 SLC 500 4-Channel Thermocouple/mV Input Module User Manual - Manual (Page 16)

Publication 1746-UM007C-EN-P - July 2004
1-4 Overview
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.
Thermocouple
Input
Module
SLC 500
Processor
Channel Configuration Word
Channel Data Word
Thermocouple or mV
Analog Signals
Channel Status Word