Plugboard

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

IBM 402 Accounting Machine plug-board wiring. This board was labeled "profit & loss summary."

An IBM 407 Accounting Machine with plug-board inserted, but not engaged.

The plugboard (steckerbrett) on the Enigma is positioned at the front of the machine, below the keys. In the photograph, two pairs of letters are swapped (S-O and J-A). Up to 13 letters can be swapped this way.

A plugboard, or control panel, was a device used to direct the operation of unit record equipment, some cypher machines, and some early computers. They consisted of a number of plugs, or jacks, into which patch cords were inserted, completing a circuit. Wiring the plugboard "programmed" the system, which operated as a sort of read only memory.

Control panels were first introduced 1906 for the Hollerith-tabulated census, earlier machines had been hard wired for specific applications. Removable plugboards were introduced with the Hollerith (IBM) type 3-S tabulator in the 1920s. Different programs could be stored on separate plugboards, and then inserted into the tabulators as needed. IBM 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..."

A IBM 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.

A plugboard was also used on the famous Enigma machine, although it was not removable. In this case the plugboard acted as a "fourth rotor" in the rotor machine's workings. Plugboard wirings were part of the "day settings" that specified which rotors to insert into which slot, and which plugboard connections to make. In practice the plugboard did not improve the security of the cypher being generated, as it did not change with every keypress, unlike the rotors.

The first version of the ENIAC computer was programed via cabling, switches and plugboards. This should not be surprising given the genesis of the machine in the US Army's ballistics labs, which was formerly a major tabulating machine user. ENIAC's cabling was later reconfigured to use the existing Function Tables data ROM memory as program ROM memory (the switches and plugboards continued to be used in the reconfigured ENIAC). Plugboards remained in use in specialty-purpose computers for some time, acting as a ROM but able to be manually reprogrammed in the field. One example is the Ferranti Argus computer, used on the Bristol Bloodhound missile, which feature a plugboard programed by inserting small ferrite rods into slots, in effect creating a read-only core memory by hand.

Many applications using unit record tabulators were later migrated to computers such as 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.