In the Dark Age before the DOS and green screens it was certainly not given that machines would be able to communicate with each other in so many different ways. In automation at that time everything was mostly PLC organized.
Usually, such structure included one large controller, kilometers of wires between: controller, sensors and actuators and eventually one PC station with HMI interface called Supervisory Control and Data Acquisition (SCADA). Simple control applications were not connected to such expensive architecture. Connectivity between controller sensors and actuators was mainly delivered through the 4 to 20mA current loop as a method of transferring the information. A typical current loop assigned a sensing range 0 to 100% to the current range between 4 to 20mA. The transmitter impressed a certain current into the loop and the receiver measured the current in the loop. After measuring the current at the receiver side internal receiver logic then determined the present level of the signal within the defined range. Same principle was used with voltage signaling. Since, one station is always the transmitter and the other one is the receiver, this is a unidirectional system.
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