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3-3GFK-0787B Chapter 3 Output Subsystem
GMR Output Handling
Unlike GMR input voting, which is done by the GMR software in the PLCs, output voting is
performed at the output block groups. To perform output voting, the blocks must be one of the
listed types, and they must be configured (with a Hand-held Monitor) to be in GMR mode.
Output Voting
A GMR output block group compares corresponding output data for each point as
received from each of the three PLCs. If all three PLCs are online, the data from at least
two must match. The block group sets each output load to match the state commanded
by at least two of the PLCs.
Outputs from 3 PLCs
PLC A
PLC B
PLC C
0
0
1
GMR Block Performs
2 out of 3 Voting
0
Single Output
Provided to
Field Device
If only two of the three PLCs are communicating on the bus and they send matching
output data for a point, the block group sets the output to that state.
If only two PLCs are communicating, the block group performs 2 out of 3 voting using
the data from the two online PLCs and the block’s configured duplex default state in
place of the offline PLC data.
If only one of the three bus controllers is present on the bus, the block group sets output
states to match the output data sent by that PLC.
If the Simplex Shutdown feature is enabled, a PLC will shut down if it determines that it
is the only PLC still operating. The timeout period before it shuts down is configured as
the next item. When the PLC shuts down and a block group is no longer receiving
output data, outputs will go to their default state or last state, as configured at each block
group.
If all PLCs are offline, the block group forces its outputs to the block’s configured default state.
The voted state of the output is available to the GMR system for monitoring purposes to
determine output discrepancies. However, the voted output state is not available to the
application program.
Duplex Default for Outputs
As mentioned, the duplex default state is used when a block determines that only two PLCs
are online. The Duplex Default state of On or Off is used by the 2 out of 3 voting algorithm
in the block group, instead of the state that would have been supplied by the third PLC.
The Duplex Default state determines whether voting will be 1 out of 2 or 2 out of 2 when
only two PLCs are providing outputs. This is explained on the next page.