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• Make sure to have the following safety circuits outside of the PLC to ensure safe system operation
even during external power supply problems or PLC failure.
Otherwise, malfunctions may cause serious accidents.
1) Most importantly, have the following: an emergency stop circuit, a protection circuit, an
interlock circuit for opposite movements (such as normal vs. reverse rotation), and an interlock
circuit (to prevent damage to the equipment at the upper and lower positioning limits).
2) Note that when the PLC CPU detects an error, such as a watchdog timer error, during self-
diagnosis, all outputs are turned off. Also, when an error that cannot be detected by the PLC
CPU occurs in an input/output control block, output control may be disabled.
External circuits and mechanisms should be designed to ensure safe machinery operation in
such a case.
3) Note that when an error occurs in a relay, triac or transistor output device, the output could be
held either on or off.
For output signals that may lead to serious accidents, external circuits and mechanisms
should be designed to ensure safe machinery operation in such a case.
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