Rockwell Automation 1746-NR4 SLC RTD/Resistance Input Module User Manual - Manual (Page 19)

Publication 1746-UM008B-EN-P - December 2006
Overview 19
System Operation
The RTD module has three operational states.
• Cycle power
• Module operation
• Error (module error and channel error)
Cycle Power
When you cycle the module’s power, the RTD module checks its
internal circuits, memory, and basic functions via hardware and
software diagnostics. During this time the module status LED indicator
remains off. If no faults are found during the diagnostics, the module
status LED indicator is on.
After the checks are complete, the RTD module waits for valid
channel configuration data from your SLC ladder logic program
(channel status LED indicators off). After configuration data is written
to one or more channel configuration words and their channel enable
bits are set by the user program, the channel status LED indicators go
on and the module continuously converts the RTD or resistance input
to a value within the range you selected for the enabled channels. The
module is now operating in its normal state.
Each time a channel is read by the module, that data value is tested by
the module for a fault condition, for example, open circuit, short
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 indicator blinks, indicating a channel error condition.
The SLC processor reads the converted RTD or resistance data from
the module at the end of the program scan or when commanded by
the ladder program. The processor and RTD module determine that
the backplane data transfer was made without error and the data is
used in your ladder program.
Module Operation
Each input channel consists of an RTD connection, which provides:
• excitation current.
• a sense connection, which detects lead-wire resistance.
• a return connection, which reads the RTD or resistance value.
Each of these analog inputs are multiplexed to one of two analog
convertors.