Activity Check In Sequence Programs For Safety Observation - Mitsubishi Electric MELSEC Q Series Programming Manual

2 SAFETY OBSERVATION FUNCTION

2.2.1 Activity check in sequence programs for safety observation

PLC CPU and Motion CPU execute an activity check to ensure that both CPUs
execute the safety observation function correctly.
One CPU checks if the counter of the other CPU is updated. If the counter is not
updated within a certain period of time, an error will occur. The time to detect error is
automatically determined depending on the PLC CPU scan time.
When the Motion CPU detects that the PLC CPU's safety signal comparison is not
operated, "Safety observation error 24" will occur. When the PLC CPU detects that the
Motion CPU's safety signal comparison is not operated, "Safety observation error 25"
will occur. In both case, the shut-off signal will be turned OFF.
PLC CPU side
Counter value
One CPU starts updating after
detecting the counterpart's CPU updates.
Motion CPU side
Counter value
Detection of
Motion CPU side
Safety observation function.
(by PLC CPU)
Shut-off signal
(From PLC CPU to
Safety signal module)
Error will occur if Motion CPU side counter update
is not confirmed within certain time.
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