Rockwell Automation 1790P-T4T0 CompactBlock LDX I/O Thermocouple Modules - Manual (Page 12)

Publication 1790-UM003A-EN-P
1-4 Overview
Internal to the module, Cold Junction Compensation (CJC) sensors are
attached to the terminal block to enable accurate readings from each
channel. These sensors compensate for offset voltages introduced into
the input signal as a result of the cold-junction where the thermocouple
wires are connected to the module.
General Diagnostic Features
Module, network, and channel LEDs help you identify the source of
problems that may occur during power-up or during normal channel
operation. The LEDs indicate both status and power. See Chapter 4,
Diagnostics and Troubleshooting , for details on power-up and channel
diagnostics.
System Overview The modules communicate to the controller or network scanner via the
DeviceNet™ or PROFIBUS network. The 1790D modules also receive 24V
dc power through DeviceNet. An external 24V dc auxiliary source is
required to power the 1790P module and all thermocouple/mV channels.
System Operation
At power-up, the module performs a check of its internal circuits,
memory, and basic functions. If no faults are found during power-up
diagnostics, the module status LED is turned on (green).
Once a channel is properly configured and enabled, the module
continuously converts the thermocouple or mV input to a value within the
range selected for that channel.
Each time the module reads an input channel, it tests the data for a fault
(over- or under-range or open-circuit condition). If it detects a fault, the
module sets a unique bit in the channel status word. See Input Data File
on page 3-2. The module sends two’s compliment binary converted
thermocouple/mV data out over the network. See Appendix B for a
description of two’s compliment binary numbers.