Fail-Safe Circuits - Omron CP1E Series User Manual

5 Installation and Wiring
5-1

Fail-safe Circuits

This section describes the fail-safe circuits that must be set up outside the CP1E.
Always set up safety circuits outside of the PLC to prevent dangerous conditions in the event of errors in
the CP1E CPU Unit or external power supply.
In particular, be careful of the following points.
Timing of Supplying Power to the PLC and the Controlled System
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 momentarily not function properly.
To prevent any malfunction, add an external circuit that prevents the power supply to the controlled sys-
tem from going ON before the power supply to the PLC itself.
Safety Circuit for Errors (Outputs Turned OFF)
When any of the following errors occur, PLC operation (program execution) will stop and all outputs
from the Units will be turned OFF.
• A CPU error (watchdog timer error) or CPU on standby
• A fatal error (memory error, I/O bus error, too many I/O points error, program error, cycle time too
long error, or FALS 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.
Precautions for Correct Use Precautions for Correct Use
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 PROGRAM mode.
Safety Circuit after a Malfunction (When an Output Remains ON)
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. Always 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.
5-2 CP1E CPU Unit Hardware User's Manual(W479)