IBM Model F

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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 keyboards are the IBM Personal Computer keyboard and the IBM Personal Computer AT keyboard, but 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 produced in the 1980s. By 1983, however, IBM had designed the much cheaper, easier to manufacture Model M keyboard 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, and were designed to replace IBM's costlier 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 consisted of a curved metal backplate and a curved, painted metal upper plate. The PCB is flexible, and 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 actually built into its base, allowing all of its keycaps to be the same shape and size, and thus easily interchangeable and customizable.

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

Model F keycaps were the same design 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 on terminals.


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.

IBM Personal Computer keyboard

The keyboard of the 1981 IBM PC 5150 is commonly referred to as the "PC XT" keyboard, although it predates the XT. It was based on the Datamaster keyboard, but was mounted in an external case and used a unidirectional serial connection with a DIN-5 connector. When the PC was released, the keyboard was acclaimed for its touch, but its layout was criticized for having many oddly shaped keys placed close to each other. The US English layout variant had the part number 1501100. The ISO German layout variant had the part number 1501102.

IBM CS/9000 Keyboard

The CS/9000 (also known as the System 9000) of 1982 was a laboratory computer based on a Motorola 68000 CPU. Its keyboard was electronically interchangeable with that of the PC and PC/XT. It's not clear whether it had the exact layout of the regular PC keyboard.

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. The US layout had part number 6450300.

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 5291 Keyboard

IBM's 5291 terminals also used 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 feet. US-layout variants had the part numbers 4176191 and 1397950.

IBM 3178 Keyboards

The IBM 3178 was a low cost 3270 terminal based on the design of the 3101 launched in 1983. Much like the 3278, it had an array of different 75-key and 87-key keyboard options, and used the same physical and functional layouts as its beam spring predecessors. The keyboard had no feet, and the 75-key variants used the same size casing as the 87-key ones, but with the right hand keypad area covered over. The label on the back of the keyboards are designated with the model of the terminal that they are intended to be used with - C1 through C4. Part numbers were of the form 56409xx depending on version.

IBM 3101 Model F Keyboard

In 1983, IBM replaced the beam spring keyboard of the 3101 with a Model F keyboard. It consisted of the 3178 keyboard with the functional layout of the 3101. This unit was electronically interchangeable with the older beam spring 3101 keyboard.

IBM 3104 Keyboard

The 3104 was a low cost mainframe terminal. It had a limited subset of the 3178's keyboard options, and they had their own part numbers. It is unclear whether or not 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. It featured a revamped keyboard design, which was probably the final Model F keyboard design before the first Model M's appeared later on in the year. The layout was modified to address criticisms of the earlier PC keyboard, and somewhat resembled the layout used on the IBM Displaywriter keyboard. The communications protocol between the keyboard and computer was also redesigned, it used a bi-directional protocol which supported the use of lock lights on the keyboard. Despite being incompatible with the older PC keyboard, it still used the same DIN-5 connector. The AT keyboard protocol became standard and is compatible with that used on PS/2 keyboards. The US layout variant had the part number 6450200.

IBM Displaywriter Model F Keyboard

In 1982, IBM made available a Model F keyboard for the Displaywriter in addition to the original Beam Spring variants. It was marketed as a "low profile" and "ergonomic" alternative to the original Beam Spring keyboards. The keyboard was only available in the 84-key, 96-character layout, and not the less common 82-key, 92-character variant. The Displaywriter Model F used the same two-stage feet as some of the terminal keyboards, and featured 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.) The PC terminal emulator keyboards are the only keyboards that actually make use of all 122 keys. Most (if not all) of the dedicated terminal keyboards have a number of blank keys which do nothing when pressed.

IBM 104-key Model F Keyboards

The 104-key Model F was a less common variant of the 122-key terminal keyboard which lacked 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 a rubber dome 104-key keyboard (part numbers 73X38xx) for some of their terminals which was manufactured by Micro Switch.

IBM 4980 Keyboard

The IBM 4980 was a terminal for the IBM Series/1 minicomputer, which used a 127-key Model F keyboard. It was otherwise similar to the regular 122-key keyboard, but featured additional keys so that the keyboard's layout was more similar to the beam spring keyboards used on older Series/1 terminals (the 122-key keyboards even have the contacts for the extra keys present on 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: Clones of the IBM Model F