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3-5GFK-0787B Chapter 3 Output Subsystem
Results of Block Group Voting with One PLC Online
If two PLCs are offline, the “voted” outputs are the same as the outputs from the PLC
which is still online (x = don’t care).
PLC A Output
State
PLC B Output
State
PLC C Output
State
Duplex Default
Setting in Block
Output State
0 x 0
0 x 0
0 x 0
0 x 0
1 x 1
1 x 1
1 x 1
1 x 1
PLC Logon Control
To prevent untripping of tripped block outputs, blocks do not use output data from a
PLC that has previously been offline until one of the following occurs:
A. all of the output data received from the newly-online PLC agrees with the voted
output data of the block.
B. the user forces the PLC to log onto the output block(s) by turning on the GMR
control bit FORCLOG (Force Logon).
For more information about PLC Logon control, please see page 7-17.
Output Fault Reporting
On detection of any block or circuit fault, a directed fault message is transferred to the
three PLCs on an event-driven basis.
The PLC currently operating as the Autotest Master also monitors output blocks for
discrepancies between the output values commanded by the PLCs. If a PLC is offline, its
data is not considered “discrepant”. But if a PLC is online and its data is discrepant, the
GMR software logs a fault into the I/O Fault Table of the PLC that detects the
discrepancy which is copied to the other PLCs. The appropriate fault references are also
set in each PLC.