IBM Model F

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IBM Model F
Part number Various (see below)
Branding IBM
Manufacturer IBM
Layouts Various
Keyswitches IBM capacitive buckling spring
Interface XT, AT, others
Years of production 1981—1994

Model F was the designation given by IBM to their buckling spring over capacitive keyboards, manufactured by IBM and Lexmark between 1981 and 1994. The best known Model F's are the IBM Personal Computer keyboard and the IBM Personal Computer AT keyboard. They were also made for a wide variety of other IBM computers and terminals, and for some of the IBM Electronic Typewriter family.

First appearing in the System/23 Datamaster of 1981, the Model F design was used in most IBM keyboards of the early to mid-1980s. By 1983, however, IBM had designed the much cheaper, more easily made Model M with its buckling spring over membrane switching system. After the AT Model F, IBM transitioned to the Model M, although certain terminal Model F variants continued to be produced as late as 1994. Unlike the Model M, the Model F designation was not always shown on the back of the keyboard. It can sometimes be found on the label on the backplate of the keyboard assembly, or it may be completely absent.

With the exception of the IBM PC AT, most Model F keyboards require an active hardware adapter to be reliably used with a modern PC.

Common design features

Model F keyboards were the first to use IBM's buckling spring key mechanism, designed to replace IBM's costlier and more complex beam spring keyboards. They were lighter and more compact, to the extent that IBM's sales literature described them as "low profile" and "ergonomic". The Model F utilized the same capacitive-contact design as the beam spring keyboards; thus many early Model F designs were buckling spring versions of beam spring keyboards, with slight differences in electronics.

The Model F's internal assembly consists of a curved metal backplate and a curved, painted metal upper plate. The PCB is flexible, and is thus also curved when attached to the backplate. Many keyboards's curves are simulated by their keycap profiles; however, the Model F's curve is built into its base, allowing all of its keycaps to be the same shape and size, thus easily interchangeable and customizable.

The springs with their attached hammers (the 'Pivot Plate Assemblies' in IBM terminology) are housed in removable barrel modules that slide into slots on the upper plate. The upper assembly plate is lined on the inside with a layer of foam. The upper and lower plates are held together with a series of interlocking tabs, with one tab from the top plate folded over the bottom plate. Depending on the keyboard variant, the modules are held in place either by a stud beneath the barrel or by a tab protruding from the barrel's bottom right side. The former is common in stabilizer-free (except for spacebar) keyboards; the latter was used in variants with stabilizer bars on the large keys.

Model F keycaps are the same as those used in the later Model M. One-piece keys were standard, but the 122-key PC 3270 and 3179 terminal keyboards were the first to use two-part keys, with detachable caps, to simplify customization.


IBM System/23 Datamaster Keyboard

The first Model F keyboard appeared on the IBM System/23 Datamaster, released one month before the IBM PC. It was an 83-key keyboard mounted internally in the Datamaster unit. Its physical and functional layout was similar to that of the IBM 5251 terminal. When the Datamaster's engineers subsequently designed the PC, they convinced their superiors to use the same keyboard, thus spawning Model F variants for a number of other IBM computers.

The original Datamaster system was known as the 5322. In 1982, IBM launched the 5324 system.[1] Amongst other differences, the newer model was not an all-in-one system, and instead had separate display, keyboard and processor units. The 5324 keyboard looks similar to the later 5291 terminal keyboard, except the legends on the keycaps are the same as that of the integrated keyboard on the original 5322 Datamaster system.

IBM Personal Computer keyboard

This is the standard keyboard bundled with the iconic IBM PC computer launched in 1981, also known as IBM 5150, and thus became the most iconic IBM Model F keyboard despite being based on the previous Datamaster design.

IBM CS/9000 Keyboard

The CS/9000 of 1982—also known as the System 9000—was a laboratory computer based on a Motorola 68000 CPU. Its keyboard was electronically interchangeable with that of the PC and PC/XT, and had a layout similar to the 5150.

IBM Portable Computer Keyboard

The 5155 Portable PC was a briefcase-style portable computer released in 1984. It used the standard PC/XT keyboard with some modifications: the keyboard attached over the front of the case when not in use. It used a 6P6C RJ-25 connector with a thinner cable, and included an adapter to DIN-5 so it could be used with a standard PC.

IBM EMR Keyboard

The EMR keyboard appears to have been used in specialized TEMPEST-hardened versions of IBM PCs designed to block electromagnetic signals that could be illicitly monitored. It appears to be a standard PC/XT Model F with a different cable and connector, and with shielding around the controller card inside the keyboard. Examples have been seen with Caps Lock located to the left of A, Ctrl located to the left of the spacebar, and Alt to the right of the spacebar. Known examples were for PC/XT-style systems, but there may have been some variants of the 122-key terminal, AT, and Model M Enhanced Keyboards with similar cable/connector modifications. The EMR keyboard used a DIN connector with a shield cable, and the EMR II keyboard used a 4-pin D-SUB connector.

IBM TPC keyboard

The TPC keyboards are very similar to the EMR keyboards mentioned above. Both PC/XT and PC/AT compatible variants have been found, with the PC/AT variant being build around the F/AT keyboard. Major differences include a shielded, uncoiled cable with a serial connector at the end, as well as copper shielding around the controller and rubber-insulated components. Despite the difference in cable and plug, the keyboard utilizes the AT protocol.

IBM 5291 Keyboard

IBM's 5291 terminals also did not use the same keyboard as the Datamaster, mounted in a larger case than the PC/XT keyboard. (For this reason, aficionados often refer to the 5291 as "Bigfoot".) It had the same functional layout as the 5251 terminal keyboard, and attached to the terminal with a 15-pin connector. An unusual feature was the keyboard's three-stage risers. US-layout variants had the part numbers 4176191[2] and 1397950.

IBM 3178 Keyboards

The IBM 3178 was a low-cost 3270 terminal based on the 3101 launched in 1983. Much like the 3278, it had an array of 75- and 87-key options, and used the same physical and functional layouts as its beam spring predecessors. It had no risers, and the 75-key variants used the same enclosure as the 87-key ones, with the number-pad area covered. The keyboards' back labels show the model of the terminal they are intended for—C1 through C4. They had part numbers 56409xx, depending on version.

IBM 3101 Model F Keyboard

In 1983, IBM replaced the beam spring 3101 keyboard with a Model F 3178 keyboard bearing the 3101's functional layout. The two keyboards were electronically interchangeable.

IBM 3104 Keyboard

The 3104 was a low-cost mainframe terminal. It had a limited subset of the 3178's keyboard options and its own part numbers. It is unclear if they were compatible with the keyboards used on the 3178.

IBM Personal Computer AT keyboard

The Personal Computer Advanced Technology (PC AT), released in 1984, was the successor to the IBM PC and XT. Its revamped design was probably the final Model F keyboard design before the first Model M's appeared later that year. The layout was modified to address criticisms of the earlier PC keyboard, and somewhat resembled the layout of the IBM Displaywriter. The communications protocol between keyboard and computer was also redesigned, with bidirectional signals supporting lock lights. Despite its incompatibility with the older PC keyboard, the PC AT used the same DIN-5 connector. The AT keyboard protocol became standard, and is compatible with that of PS/2 keyboards, enabling it to be used with PS/2 connections via a simple DIN-to-PS/2 adapter. The US layout variant has the part number 6450200.

IBM Displaywriter Model F Keyboard

In 1982, IBM made a Model F keyboard available for the Displaywriter, marketing them as "low profile, ergonomic" alternatives to the original beam spring keyboards. It was available only in the 84-key, 96-character layout, and not the less common 82-key, 92-character layout. The Displaywriter Model F used the same two-stage risers as some of the terminal keyboards, and had an all-white key colour scheme.

IBM 122-key Model F Keyboards

The 122-key keyboards used originally for the PC 3270 (an XT with terminal emulation software and hardware) and later the 3179 and 3180 terminals. Part number 611034x, depending on version, layout, and whether it was for use with an emulator or actual terminal. It used a 240-degree DIN-5 connector. The layout was arguably inspired by the DEC LK-201, and seems to have been the basis for the layout of the later Model M Enhanced Keyboard. They can be distinguished from the Model M 122-key keyboards by their black metal underside (as opposed to plastic on the Model M equivalents). Unlike other dedicated terminal keyboards, which have a number of blank keys that do nothing, the PC terminal emulator keyboards actually make use of all 122 keys.

IBM 104-key Model F Keyboards

The 104-key Model F was a less common variant of the 122-key terminal keyboard without a numeric keypad. It was primarily associated with two types of IBM terminal – the IBM 5085 Graphics Display (part number 6016730), and the IBM 3290 Display Station (part number 1387033). The variant for the 5085 had a speaker two D-SUB connectors at the back – one to connector to the terminal, and the other to interface with other peripherals for the 5085. The 3290 variant used the regular 240-degree DIN-5 connector seen on most IBM terminals of the time and lacked a speaker. IBM later offered rubber dome versions of the 104-key keyboard (part numbers 73X38xx and 09F4230) for some of their terminals; they were manufactured by Micro Switch.

IBM 4980 Keyboard

The 4980 was a terminal for the IBM Series/1 minicomputer, which used a 127-key Model F keyboard. Based on the regular 122-key keyboard, it had additional keys that made its layout more similar to the beam spring keyboards on older Series/1 terminals. (122-key Model F's have contacts for the extra keys of the 127-key layout.)


The design and layout of IBM Model F—in particular the AT version— was widely copied by other keyboard manufacturers in both the US and the Far East.

See also