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4-5GFK-0787B Chapter 4 PLC Subsystem
CPU Sweep in a GMR System
The special functions required for Genius Modular Redundancy include autotest, input
voting, and alarming. These GMR functions are provided by a set of Program Blocks that
are placed into the Program Folder using the LM90 librarian feature. After this is done,
the GMR logic is executed automatically by the CPU as shown below.
Start of Sweep
Housekeeping
Input Scan
GMR functions
Application Program
GMR functions
Output Scan
Additional CPU Tasks
PLC Operation
Each PLC in the GMR system receives the input state from each connected block on each
PLC sweep.
The GMR software performs any input voting required for both discrete and analog
inputs and provides voted input data to the PLC. It notes any data discrepancies and
provides fault bits and fault messages that can be accessed by application program.
As always, the application program determines the required state of the outputs as a
function of the inputs received. The application program sets a single output bit for each
device to be controlled. The appropriate number of redundant Genius blocks are
configured to identical output references.
The CPUs monitor the voted output state computed by each Genius output block group
and provide diagnostic information on the detection of any output discrepancy and
identifies the discrepant PLC.
The executive path in each processor (field input to field output) is independent of any
inter-processor data exchange, with the exception of initialization data at powerup.