3-5SectionFail-safe Circuits
39
It is possible for an output to remain ON due to a malfunction in the internal cir-
cuitry of a contact output, such as a transistor failure. Provide any circuits neces-
sary outside of the PC to ensure the safety of the system in the event that a con-
tact output fails to turn OFF.
When the PC output controls opposite operations, such as forward and reverse
operation of a motor, or in cases where incorrect PC operation may cause an
accident or damage to equipment, set up interlock circuits outside the PC. An
example is given below.
Interlock circuit
Forward rotation of motor
Reverse rotation of motor
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.
Contact Output Failures
Interlock Circuits
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