GE GFK-0787B Discrete Output Discrepancy Reporting, How Output Discrepancy Checking is Performed

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Discrete Output Discrepancy Reporting

Output discrepancy monitoring is the process of monitoring the block output voting function to detect both processor discrepancies and lost communication between the block and the other processors. All PLCs periodically monitor all blocks' discrepancy status. On interrogation by any PLC, the block responds with a discrepancy message indicating the discrepant output and disagreeing PLC.

The system uses output discrepancy checking to determine if the output data sent from each of the PLCs agrees with the voted output state. If a discrepancy check reveals that a PLC is sending incorrect output data to a block, the GMR system logs an output discrepancy fault in the I/O fault table and sets the appropriate fault contacts.

The GMR system performs output discrepancy checking whenever it is not performing input or output autotesting (i.e. between autotests during the autotest interval). It checks all output blocks in redundant output groups and any non-redundant output blocks marked for discrepancy checking in the GMR configuration.

How Output Discrepancy Checking is Performed

If the GMR system determines that an output changed state during a discrepancy check, it attempts up to three times to properly complete the discrepancy check on an output block. This prevents logging false discrepancy faults that might be caused by the application program changing the state of an output while a discrepancy check is being performed

Discrete Output Discrepancy Reporting with Dynamic Outputs

Output Discrepancy Checking gives valid results as long an output changes state less frequently than approximately once per 10 PLC scans. If an output changes state more rapidly than approximately once per 10 PLC scans, the results of Output Discrepancy Checking may be ignored. Nuisance discrepancy faults (caused by transitioning outputs) should NOT ever be logged. However, a message is logged in the PLC fault table. The message indicates that output discrepancy processing could not be completed for a device at rack X, slot Y, SBA x due to transitioning outputs.

In an ESD system, outputs are normally static. Outputs that are not static, that is, outputs that normally change state, may not be autotested as frequently as expected.

5-12	GeniustModular Redundancy Flexible Triple Modular Redundant (TMR) System	GFK-0787B
	User's Manual ±March 1995