Supervisory control apparently got its start sometime about 1920. According to available history on the subject, an engineer from a switch gear builder attended an AIEE meeting in March, 1920. At that time most of the electric utility sub-stations were under automatic control, and the young engineer was questioned about the adequacy of this. He admitted that it had some shortcomings and also that remote control equipment could be developed patterned on telephone switching circuits. One customer at the meeting stated that this was just the kind of equipment his system needed and asked when he could get some. This put the wheels in motion; the switch gear put the wheels in motion; the switch gear builder contacted an independent telephone builder, and the first supervisory system was developed. At approximately the same time, another switch gear builder also went to an independent telephone manufacturer and, borrowing some of their techniques and equipment, came up with a similar system. Each could be described as a select check-before-operate system, operating in a quiescent mode.
A quiescent system has no parts or circuits in motion when at rest. In this kind of system, a point may be addressed either from the master station, where the address is set up by momentarily operating the select key, or from the remote station, where the breaker trips the auxiliary switch contact, delivers a signal to the supervisory, and generates an address that is transmitted to the master station. The master station then puts the point number in storage and confirms the address code back to the remote station. Then the remote station transmits the new status or indication to the master, updating the point escutcheon. When the address is generated at the master station, the point escutcheon starts the master station in point escutcheon starts the master station in motion; the address is transmitted to the remote station, put in storage, and reconfirmed to the master station creating the select check operation. To control the operation remotely, common trip and close pushbuttons generally re used to open or close the breaker, transmitting a separate code different from a select code.
It is hard to tell how many of these systems were built over the years, but they were numerous, and they have provided excellent service. Many of them are still in operation.
Until less than 10 years ago, the supervisory control business moved along practically unnoticed; then, the advent of solid state techniques began to disturb it. Solid state techniques employ a continuous scanning principle in generating the codes that transmit the principle in generating the codes that transmit the information from one station to another. In generating these discrete codes certain error detection methods can be used to provide greater security. when a blast of noise eliminates a code, the code generally is repeated until a channel is clear and the information is received. The receiving station is thus updated. In some of the older quiescent systems, a message could be obliterated by noise blast; because many remote units of this early design did not speak more than once, the information was permanently lost. Using the scanning principle, supervisory designers were able to principle, supervisory designers were able to develop larger systems in which one master station could handle many remote stations. In this case, it would be basically an interrogation scanning system whereby the remote stations are normally quiescent but the master station sequentially interrogates each remote for a message to transmit. If there is one, it sends a complete scan of station information to the master. The master then moves to the next remote requesting the same information. This process continues and repeats until all remote stations are polled and any transmitted information is updated in the master.
Because solid state techniques made quiescent systems virtually obsolete, these systems will be retired for the purposes of this paper. Also, in the larger interrogation scanning systems, they are in the minority when it comes to a general coverage of a supervisory control.