Plugboard
| Industry | Computing hardware, unit record equipment |
|---|---|
| Founded | 1911 |
| Headquarters | Armonk , United States |
| Products | Control panels (plugboards) for unit record equipment and early computers |
A plugboard—also known as a control panel depending on the context—is a matrix of jacks or sockets (often called hubs) into which patch cords are plugged to establish electrical circuits. These control panels managed operations in unit record equipment, cipher devices, and early computing machines. The flat, detachable panel slots into the device, allowing quick reconfiguration to handle varying functions.
Contacts within the machine connect to elements such as relays, counters, card reader inputs, card punch outputs, or printers. By wiring the plugboard, these components are linked to carry out particular tasks like reading punched cards and totaling numerical data. This mechanism is comparable to a field-programmable gate array (FPGA), where the plugboard acts as the customizable wiring layer.
Unit record equipment
Initially, machines were hardwired for fixed purposes until Otto Schäffler introduced the plugboard concept, enabling tabulators to be easily reprogrammed.[1] Different applications could be set up on separate control panels and inserted as required. The use of removable control panels became standard practice in unit record equipment that needed rewiring for various jobs.
IBM produced removable control panels ranging in size from 6¼" by 10¾" (used in models such as IBM 077, IBM 550, IBM 514) up to roughly one to two feet square (300 to 600 mm), featuring rectangular arrays of hubs.[2] Single-wire patch cords with plugs at both ends connected output hubs to input hubs inside the machine. For instance, during card duplication, a hub reading a card column could be wired to a punch magnet hub, allowing selective copying or omission of fields. Complex tabulator setups might require dozens of patch cords.
Tabulator operations combined mechanical and electrical parts. While control panels simplified rewiring electrical circuits, most machines still demanded mechanical adjustments for new functions. The IBM 407 marked the first IBM tabulator fully controlled electrically through its control panel and carriage tape, eliminating the need for mechanical modifications.
IBM documentation refers to control panels as devices that "direct" or facilitate "automatic operation" in collators, interpreters, and the IBM 407. Calculator control panels on models like the IBM 602 and IBM 604 outlined sequences of operations and were described as programs.
Wiring of unit record equipment control panels
Unit record devices were programmed for specific functions by inserting removable control panels. These panels provided the electrical connection points of machine components; wiring defined how these elements interconnected once locked into place. This approach is analogous to modern field-programmable gate arrays, where fixed logic blocks are user-configured through wiring.
Designing a control panel’s wiring required understanding both machine components and timing requirements. Components operated in sync with a rotating shaft defining a machine cycle—advancing cards or printing lines and totals. Each cycle was divided into points corresponding to punched card rows under read or punch stations. Cards typically fed face down with the 9-edge first; cycle points ranged from 9-time down through 0-time (digits), followed by 11 and 12 time (zones).
At read stations, there were 80 spring wire brushes, one per column, pressing against cards. When a hole passed beneath a brush, it contacted a conductive surface connected to power, generating an electrical pulse or impulse. Each brush connected to an individual hub on the control panel for wiring flexibility. The timing of impulses within the cycle determined their function; for example, an impulse at 7-time on a wire connected to column 26’s punch magnet would cause punching row 7 in that column. Zone and digit impulses combined on single wires were separated by relay circuits based on timing intervals.
Control panels arranged exit (output) and entry (input) hubs logically. Several adjacent common hubs permitted multiple wires per connection point. Some bus hubs linked multiple wires without internal circuitry; small wire splits joined three or four wires above the panel.
Unit record machine components evolved over many years and differed among machine models. Typical hub groupings on later IBM equipment included:
- Read brushes: 80 output hubs per read station; tabulators could have multiple stations; reproducing punches often featured verification read stations.
- Punch magnets: input hubs per card column activating electromagnets to punch holes.
- Print entries: one hub per print position controlling print bars or wheels; certain machines wired print outputs to counters ensuring printed totals matched calculations.
- Counter entries: various counters with input hubs for addition or subtraction using digit impulses; counters supported carry operations and coupling for longer numbers; subtraction employed nines complement arithmetic.
- Counter total exits: output hubs emitting total pulses wired to print positions; after printing totals, counters reset; special circuits handled negative values with appropriate printed symbols.
- Comparing: circuits with two inputs and one output emitted pulses when inputs differed; some identified higher values; implemented as XOR gates with opposing electromagnets.
- Distributors: allowed one output pulse wired simultaneously to multiple inputs without feedback loops.
- Emitters: sets of 12 output hubs generating pulses at specific cycle times via rotary switches; used for numeric constants like dates; later models had alphanumeric emitters requiring only one wire.
- Selectors: routed pulses from common inputs to two outputs based on energized relay magnets; included immediate and pilot selectors with delayed activation; co-selectors had multiple contacts triggered by pilot selectors.
- Digit selectors: functioned like emitters but switched entry hubs through outputs during cycles; capable of detecting specific digits by wiring read brushes and pilot selectors.
- Column splits: relays energized only at 11 and 12-time separating digit from zone pulses.
- Storage: later models such as IBM 407/602 mechanically stored values via sliding contacts controlling impulse emission timing until cleared.
This wiring framework allowed flexible programming of unit record machines tailored for a wide range of business data processing needs.
Cypher machines
The plugboard on the Enigma machine acted as a "fourth rotor" within the encryption mechanism of this rotor machine. Its wiring configurations formed part of the "day settings," which dictated both rotor arrangement and plugboard connections. Although it contributed to strengthening the cipher, its effect was somewhat limited because it did not alter with every keystroke as the rotors did. Refer to Cryptanalysis of the Enigma for further details.
Early computers
Programming the original ENIAC computer involved the use of cabling, switches, and plugboards. Subsequently, its cabling was modified to utilize Function Tables data ROM as program ROM, yet switches and plugboards remained integral to its programming approach.
The IBM 305 RAMAC incorporated a plugboard to manage program compare and branch functions, alongside additional plugboards that controlled card reading, punching, printing, and console typewriter operations. Numerous peripherals for early IBM systems—including the IBM 700/7000 series, IBM 650, IBM 711, and IBM 716—derived from unit record machines equipped with plugboards.
Plugboards continued to be employed in specialized computers as manually reprogrammable read-only memory (ROM). For instance, the Ferranti Argus computer used in the Bristol Bloodhound missile system featured a plugboard that was programmed by placing ferrite rods into designated slots, effectively producing a hand-configured core memory.
See also
- Enigma machine
- Powers-Samas, a British producer of unit record equipment utilizing removable "connection-boxes" with mechanical linkages rather than plugboards
- Telephone switchboard
- Breadboard, a solderless plugboard designed for electronics prototyping