Spectrum Controls 1769sc-IR6I - Manual (Page 12)

1-4 Compact™ IO Isolated RTD/Resistance Module
User’s Manual Pub. 0300241-01 Rev. A
1.1.5 General Diagnostic Features
The module contains a diagnostic LED that helps you identify the source of problems that
may occur during power-up or during normal channel operation. The LED indicates both
status and power. Power-up and channel diagnostics are explained in Chapter 5,
Diagnostics and Troubleshooting.
Section 1.2
System Overview
The modules communicate to the controller through the bus interface. The modules also
receive 5 and 24V dc power through the bus interface.
1.2.1 System Operation
At power-up, the 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 power-up diagnostics, the module status LED is turned on.
After power-up checks are complete, the module waits for valid channel configuration
data. If an invalid configuration is detected, the module generates a configuration error.
Once a channel is properly configured and enabled, it continuously converts the RTD or
resistance input to a value within the range selected for that channel.
Each time a channel is read by the input module, that data value is tested by the module
for an over-range, under-range, open-circuit, or “input data not valid” condition. If such
a condition is detected, a unique bit is set in the channel status word. The channel status
word is described in Input Data File on page 4-9.
Using the module image table, the controller reads the two’s complement binary
converted RTD or resistance data from the module. This typically occurs at the end of
the program scan or when commanded by the control program. If the controller and the
module determine that the data transfer has been made without error, the data is used in
the control program.
1.2.2 Module Operation
As shown in the block diagram below, each input channel of the module consists of an
RTD/resistance connection that accepts excitation current (i.e. current out); a sense
connection that detects lead wire resistance; and a return connection. Each channel has
an A/D converter that reads the RTD or resistance value and the lead wire resistance.