Allen-Bradley MicroLogix 1200, User Manual, Page: 14 / 144

Publication 1762-UM002A-EN-P - July 2002
1-4 Overview
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 4,
Diagnostics and Troubleshooting.
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.
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 thermocouple or
millivolt 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 3-2.
Using the module image table, the controller reads the two’s
complement binary converted thermocouple or millivolt 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.