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Fail-safe Circuits Section 5-1
5-1 Fail-safe Circuits
Be sure to set up safety circuits outside of the PLC to prevent dangerous con-
ditions in the event of errors in the PLC or external power supply.
Supply Power to the PLC
before Outputs
If the PLC’s power supply is turned on after the controlled system’s power
supply, outputs in Units such as DC Output Units may malfunction momen-
tarily. To prevent any malfunction, add an external circuit that prevents the
power supply to the controlled system from going on before the power supply
to the PLC itself.
Managing PLC Errors When any of the following errors occurs, PLC operation will stop and all out-
puts from Output Units will be turned OFF.
• Operation of the Power Supply Unit’s overcurrent protection circuit
• A CPU error (watchdog timer error) or CPU on standby
• A fatal error* (memory error, I/O bus error, duplicate number error, too
many I/O points error, program error, cycle time too long error, or
FALS(007) error)
Be sure to add any circuits necessary outside of the PLC to ensure the safety
of the system in the event of an error that stops PLC operation.
Note *When a fatal error occurs, all outputs from Output Units will be turned OFF
even if the IOM Hold Bit has been turned ON to protect the contents of I/O
memory. (When the IOM Hold Bit is ON, the outputs will retain their previous
status after the PLC has been switched from RUN/MONITOR mode to PRO-
GRAM mode.)
Managing Output
Malfunctions
It is possible for an output to remain ON due to a malfunction in the internal
circuitry of the Output Unit, such as a relay or transistor malfunction. Be sure
to add any circuits necessary outside of the PLC to ensure the safety of the
system in the event that an output fails to go OFF.
Emergency Stop Circuit The following emergency stop circuit controls the power supply to the con-
trolled system so that power is supplied to the controlled system only when
the PLC is operating and the RUN output is ON.
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