Omron SYSMAC CS Series Operation Manual page 77

Fail-safe Circuits
Supply Power to the PLC
before Outputs
Unit Errors
Contact Output Failures
Interlock Circuits
turn OFF. Take any safety measures necessary outside of the Unit to
ensure the safety of the system in the event that outputs turn OFF.
If the PLC's power supply is turned ON after the controlled system's power
supply, contact outputs may malfunction momentarily and, as a result, the
controlled system's outputs may operate incorrectly for a short time. To pre-
vent 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.
When any of the following fatal errors occur at the Customizable Counter Unit,
the Unit will stop operation (and processing) and all contact outputs and pulse
outputs will be turned OFF:
• A Unit WDT error, a Unit RAM error, a memory error, a no END instruction
error, a FALS instruction execution, or a CYCLE TIME OVER error
Note For analog outputs, either the value set (maximum value, present value, clear)
with the output hold function in the Unit Setup Area (DM 6614) or 0 V will be
output. (For details, refer to 7-7 Analog Outputs.)
Set up safety circuits outside of the Customizable Counter Unit to prevent
dangerous conditions in the event of the above errors.
It is possible for an output to remain ON due to a malfunction in the internal
circuitry of a contact output, such as a transistor failure. Provide any circuits
necessary outside of the PLC to ensure the safety of the system in the event
that a contact output fails to turn OFF.
When the PLC output controls opposite operations, such as forward and
reverse operation of a motor, or in cases where incorrect PLC operation may
cause an accident or damage to equipment, set up interlock circuits outside
the PLC. An example is given below.
PLC
In the above example, if outputs IR 00100 and 00102 both turn ON together
(incorrect operation), the interlock circuit will stop MC1 and MC2 turning ON
together.
Interlock circuit
Forward rotation of motor
Reverse rotation of motor
Section 3-5
57