Plug-board

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This article is about plug boards, or control panels, in unit record machines and early computers. For other uses, see plug board.

A plug-board, or, more formally, a control panel, was a device used to direct the operation of unit record equipment (and some early computers) built by IBM and other companies during the punch card era. Control panels were first introduced 1906, earlier machines were hard wired for specific applications. Removable control panels were introduced with the Hollerith (IBM) type 3-S tabulator in the 1920s. (IBM unit record operation manuals use the word "program" only for calculators, such as the IBM 602 and IBM 604, that executed a sequence of operations. For all other machines, from sorters, interpreters, to the IBM 407, the control panel "directed" or "automatic operation was obtained by...")

Each machine type had its own control panel configuration, specific to that machine's functions. A control panel was roughly one to two feet (300 to 600 mm) on a side and had a rectangular array of holes or hubs. Pins at each end of a jumper wire were inserted into hubs, making a connection between two contacts on the machine when the control panel was placed in the machine, thereby connecting an emitting hub to an accepting hub. For example, in a card duplicator application a card column reading (emitting) hub might be connected to a punch column (accepting) hub. It was a relatively simple matter to copy some fields, perhaps to different columns, and ignore other columns by suitable wiring. Tabulator control panels sometimes required dozens of such jumpers for complex applications. Proper wiring of a control panel required a knowledge of the electromechanical design and timing of each machine type.

Different length wires were of different colours, to help prevent dense piles of wires from building up. Wires could also be temporary or permanent - permanent wires had little latches on the ferrules, and required a special tool to remove them. These were typically used once a board had been debugged.

Removable control panels had the advantage over older approaches of being able to be setup without having to be in the unit record machine room or turning off the machine for long periods of time. Further, a library of control panels could be built and swapped in and out of the machine as needed. One control panel might be used for the weekly payroll, another for monthly reports, for example.

Many applications using unit record tabulators were later migrated to computers such as the the IBM 1401. Two programming languages, FARGO and RPG, were created to aid this migration. Since tabulator control panels were based on the machine cycle, both FARGO and RPG emulated the notion of the machine cycle and training material showed the control panel vs. programming language coding sheet relationships.