GE Mark VIe System Manual page 54

1.7.8.1 Median Value Analog Voting
The analog signals are converted to a floating-point format by the I/O pack. The voting operation occurs in each of the three
controller modules (R, S, and T). Each controller receives a copy of the data from the other two channels. For each voted data
point, the controller has three values including its own. The median value voter selects the middle value of the three as the
voter output. This is the most likely of the three values to be closest to the true value.
Median Value Voting Examples with Normal and Bad Inputs
1.7.8.2 Two Out of Three Logic Voter
Each of the controllers has three copies of the data for the logic voter. Voting is a simple logic process, inputting the three
values and finding the two values that agree.
1.7.8.3 Disagreement Detector
A disagreement detector continuously scans the input prevote input data sets and produces an alarm bit if a disagreement is
detected between the three values. Any disagreement between the prevote logical signals generates an alarm. Failure of one of
the three voted input circuits has no effect on the controlled process since the fault is masked by SIFT. Without a
disagreement detector, a failure could go unnoticed until a second failure occurs.
For analog signals, comparisons are made between the voted value and each of the three prevote values. The delta for each
value is compared with a user programmable limit value. The limit can be set as required to avoid nuisance alarms, but give
indication that one of the prevote values has moved out of normal range. Each controller is required to compare only its
prevote value with the voted value; for example, R compares only the R prevote value with the voted value. Nominal, analog
voting limits are set at a 5% adjustment range, but can be configured to any number for each analog input.
1.7.9 UDH Fault Tolerance — Command Action
Using IONet connectivity, the controller copies command traffic from the UDH across all controllers. This provides fault
tolerance for dual UDH networks.
54 GEH-6721_Vol_I_BP GEH-6721_Vol_I Mark VIe and Mark VIeS Control Systems Volume I
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