Mitsubishi Electric MELSEC iQ-R Series User Manual page 124

An input signal does not turn on (AC input module).
Cause Around the zero cross voltage (1) of the input signal (AC), there are step-like deformations as shown below:
(1)
Action Improve the input signal waveform by using an on-line type UPS and others.
An unintended signal is inputted.
Cause Noise is taken as input data.
Action Set the input response time longer.
Page 115 Input Response Time Setting
(example) 1ms  5ms
If changing the setting of the input response time has no effect, take the following two measures:
• To prevent excessive noise, avoid installing power cables together with I/O cables.
• Connect surge absorbers to noise-generating devices such as relays and conductors sharing the same power supply as noise reduction
measures.
9 TROUBLESHOOTING
122
9.2 Input Circuit Troubles and Corrective Actions