I work on water treament project 2 months in the water treament plant I can't understand How to signal sent from device(sensor,relay) to master control panel, When I read the specification I just know a process such as when water level more than specification purpose the signal from level sensor shall be sent to control panel and than control panel sent signal to stop a motor.

I never know whether what inside the control panel consist of and signal from some device should be sent analog signal or digital signal.