Rockwell Automation 1746-NR8 SLC 500 RTD/Resistance Input Module User Manual - Manual (Page 18)

Publication 1746-UM003A-EN-P
1-8 Overview
Power-up
At power-up, the RTD module checks its internal circuits, memory, and basic
functions via hardware and software diagnostics. During this time, the module
status LED remains off, and the channel status LEDs are turned on. If no
faults are found during the power-up diagnostics, the module status LED is
turned on, and the channel status LEDs are turned off.
After power-up checks are complete, the RTD 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 bits are set by the user program, the channel
status LEDs 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 flashes, 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 an analog converter.
The A/D converter cycles between reading the RTD or resistance value, the
lead wire resistance, and the excitation current. From these readings, an
accurate temperature or resistance is returned to the user program.
The RTD module is isolated from the chassis backplane and chassis ground.
The isolation is limited to 500V ac. Optocouplers are used to communicate
across the isolation barrier. Channel-to-channel common-mode isolation is
limited to ± 5 volts.
LED Status
The illustration below shows the RTD module LED panel consisting of nine
LEDs. The state of the LEDs (for example, off, on, or flashing) depends on the
operational state of the module (see table on page 1-9).