GE GFK-0787B user manual GMR Output Handling, Output Voting, Duplex Default for Outputs

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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		Single Output
PLC A	0		Provided to
		Field Device
PLC B	0		0
PLC C	1

GMR Block Performs

2 out of 3 Voting

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.

GFK-0787B Chapter 3 Output Subsystem 3-3