TMR Configuration and Voting Logic

When used in a Triple Modular Redundant (TMR) configuration, the 3500/42 Monitors and Proximitor/Seismic TMR I/O Modules must be installed adjacent to each other in groups of three. This arrangement ensures proper synchronization and fault tolerance.
In this configuration, two types of voting mechanisms are employed to guarantee accurate operation and prevent single-point failures.

1. Relay Voting (2-out-of-3 Voting)

The first level of voting occurs within the TMR Relay Module.
In this voting process, the alarm outputs from the three monitors are compared using a 2-out-of-3 logic.

• The relay is activated only when at least two monitors agree on the alarm condition.

• This voting method significantly enhances reliability by minimizing false trips due to single module faults.

For detailed information on relay voting, refer to the 3500/32 & 3500/34 Relay Module Operation and Maintenance Manual.

2. Comparison Voting

The second level of voting, known as Comparison Voting, compares the proportional value outputs of the three monitors in the TMR group.

• If the output from one monitor deviates from the other two by more than a specified threshold, that monitor automatically logs an entry in the System Event List.

• This mechanism ensures continuous monitoring of data consistency among the three channels.

Configuration Parameters:

• Comparison:
  Defines which proportional value of the TMR monitor group is used to assess deviation among the three monitors.
  It determines the acceptable range before a discrepancy is flagged in the System Event List.

• % Comparison:
  Specifies the maximum allowable percentage difference between the middle value (median) of the three monitors and each individual monitor’s value.

Both parameters are configured in the Rack Configuration Software.

TMR Input Configuration Options

For TMR applications, two types of input configurations are supported:

1. Bussed Configuration

   • Utilizes a single non-redundant transducer.

   • The signal is distributed to all three modules in the TMR group through a 3500 Bussed External Termination Block (ETB).

   • This setup simplifies wiring but does not provide input-level redundancy.

2. Discrete Configuration

   • Requires three fully redundant transducers at each measurement location.

   • Each transducer input is connected to a separate 3500 External Termination Block.

   • This configuration provides the highest level of input redundancy and fault isolation.