These modules, typically seen in line following robots, are ideally suitable for white/black line detection. You can also see them in certain entry alarm systems. HW-006 Module – A Brief Overview The ...

If your system had four such inputs, then the wiring diagram would again show an array of input devices displayed in ladder fashion. But this time, they wouldn’t share L1. Instead, each would be ...

If they do, the problem is on the processor side of the input module. Next, watch the PLC logic for a corresponding change. If it doesn't happen, then the signal isn't making it from the input module ...