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7-17GFK-0787B Chapter 7 Programming Information
Prior to sending the outputs, the application program may check the status flags. If any
are found to be 1, the application program may decide to process the initialized data
before continuing.
When the application program has computed valid outputs that can be sent to output
blocks, the application program must set control bit %M12257 (CONTINUE) to 1. When
this is done, outputs to the blocks are enabled.
If outputs fail to be enabled successfully, the GMR Software sets the System Fault status
flag (%M12235) to 1.
PLC Logon Control
PLC Logon Control prevents output states from inadvertently changing state when a
newly-initialized PLC is put online by the application program. (The application
program turns on the Continue bit: %M12257). Without PLC Logon Control, outputs
have the potential to change state if a PLC just coming online has output states that
differ from those of other PLCs that are already online, due to the output voting done by
each Genius output block group.
PLC Logon Control causes the output states from a PLC that has just come online to be
compared with the voted output states at each output block group. If the states do not
agree for any output block, the block ignores the new output data and effectively keeps
the new PLC offline with respect to that output block. This condition continues until
either the voted output states match for the complete output block or until the Force
PLC Logon control bit (%M12263, FORCLOG) is turned on. A GMR status bit (%M12240,
LOGONFT) is available. That bit indicates if this condition exists with one ore more
output blocks. It is the responsibility of the application program to monitor the
LOGONFT status bit and to turn on the FORCLOG control bit if desired, to cause output
block groups to vote on and respond to output data from all online PLCs.
Note that, if set, the LOGONFT status bit remains set until the I/O fault table is cleared,
by using the IORES control but (%M12258).
Typically, the FORCLOG and IORES control bits are set through the application program
via an operator interface or simple pushbutton wired to an input circuit.
Powerup Note
PLC Logon Control is also in effect for the first PLC in the GMR system to come online.
The first PLC to come online has its output states compared with the voted outputs
currently present at the output block groups. Remember that the output states of each
output block, with no PLCs online, are determined by the output default configuration
(0, 1, or hold last state) for each individual output circuit at each output block. For
example, if output defaults are set to Off (0) and a PLC is put online with the same
outputs already driven to On (1) states, the output block keeps the PLC offline until the
driven output states agree or until the FORCLOG control bit is set, to force the PLC
online with respect to the output block.
Performing I/O Fault Reset
It is very unlikely, but possible, that I/O faults would occur during the initialization
(powerup or stop/start cycle) of one of the GMR CPUs. Faults occurring during the
initialization of a GMR CPU are reported to that CPU. Therefore it is recommended that