Digital output signal means contact output signal expressed “1 / 0” transmitted from PLC/DCS to an auxiliary relay on the control center panel or marshalling cabinet.
For example, “on/off” signal for equipment is regarded as digital output signal. Such as solenoid valves, lamps, digital command signals to motor control center.
Link systems with PLC are widely applied for transmitting the signal, with which analog signal is converted to digital signal and is transmitted via coaxial cables or optical fibres.