Model F keyboard

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The Model F was a series of computer keyboards produced from 1981–1994 by IBM and later Lexmark.[1] Its mechanical-key design consisted of a buckling spring over a capacitive PCB, similarly to the later Model M keyboard that used a membrane in place of the PCB.

The Model F first appeared with the IBM System/23 Datamaster all-in-one computer. It is best known as part of the IBM Personal Computer in 1981 with some keycap label differences, and its subsequent release with the IBM Personal Computer/AT, where it was reconfigured with the AT protocol and some layout revisions.[2][3]

The capacitive design is widely considered superior to that of the later membrane design used on the Model M.[citation needed] It has a lighter actuation force of about 60g, a crisper feel and louder feedback, and is more robust. It also has a higher MTBF of over 100 million keypresses, and full n-key rollover.

Variations[edit]

Many IBM keyboards were based on Model F technology, featuring different keycaps, layouts and connections:

Name PN Description Image
Model F 'XT' 1801449 Released in 1981 with the original IBM PC. Uses an XT connector. 83-key PC/XT keyboard
Model F 'AT' 6450200 (US version) [4] Released in 1984 with the IBM PC AT. Uses an AT connector. 84-key PC/AT keyboard
Model F '122-key terminal keyboard' 611034x Terminal keyboard released for the IBM PC 3270. Uses an IBM proprietary terminal connector. 122-key keyboard
Model F '104-key terminal keyboard' 1387033 Terminal keyboard released for the IBM 5085 and IBM 3290.[5]
Model F '4704 62-key' 6019284 62-key keyboard released for the IBM 4704[6]
Model F '4704 107-key' 6020218 107-key keyboard released for the IBM 4704
Model F '4704 50-key' 6019273 50-key keypad released for the IBM 4704
Model F 'Displaywriter' ? Keyboard released for the IBM Displaywriter System,[7] these Model F keyboards featured fully white keys.

Modern re-creation[edit]

As of 2016–2017, a project is underway to manufacture for sale keyboards with a faithful re-creation of the model F mechanism, completely independent of IBM.[8][9][10]

Design[edit]

Model F capacitive pads from an opened assembly
Corrosion occurring on a Model F 122 terminal keyboard along with dust

The Model F's key-switch design was more durable than IBM's previous beam-spring mechanism, which was prone to failure from debris and was more complex to manufacture and service.[11] The spring assembly consisted of a top metal plate with cut holes where the plastic spring barrels reside; a bottom sheet of metal then holds the assembly together and compresses the contact sheet with a foam spacer. Earlier Model F keyboards cannot have their space bars removed without disassembling the internal assembly, this also causes a slightly different feel response from the space bar specifically: some enthusiasts modify the tension of the stabilizer on these early Model F keyboards to provide a more satisfactory response.[12]

The top metal plates in Model F keyboards are prone to corrosion and the internal foam can also rot from age, which often requires cleaning and a coating to prevent further corrosion. All Model F internal assemblies are held together with metal tabs, unlike the Model M which uses melted rivets requiring more rivets to be melted on or modified with bolts.[13]

A characteristic feature of the Model F is a plastic top shell painted with a cream paint to create a rough texture. The later Model M keyboards used injection plastic rather than paint to achieve this texture. The plastic used in the Model F is quite brittle and prone to hairline cracks, and the paint can wear off from excessive use.

Controller modifications[edit]

It is possible to use a programmable micro-controller to connect to a Model F controller and convert it to a USB-capable device for unlimited rollover, along with modifying the layout to ANSI. These exist both as external converter boxes,[14] and complete replacements of the internal electronics.[15]

Comparison with Model M[edit]

A comparison between a Model F spring (left) and a Model M spring (right); the Model M uses a spring with fewer windings and smaller plate for the plastic membrane underneath.

Although the Model F and Model M are both based on buckling-spring technology, there are considerable differences between them:

Model F Model M
External chassis Painted plastic (zinc metal in the 4704 series) and steel metal back panel (plastic in the F AT and zinc metal in the 4704 series) Molded (unpainted) plastic, plastic back panel
Internal stabilizer Only in early models
Buckling spring implementation Capacitive plate Plastic membrane
Key rollover Unlimited 2-key rollover[16]
Assembly method Reusable metal tabs Single-use rivets
Spring barrels Individually inserted in a metal plane Single plastic mold with predefined barrels

See also[edit]

References[edit]