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, this is similar to the later Model M that used a membrane in place of the PCB.

The Model F made its debut with the IBM System/23 Datamaster all-in-one computer. However, it is best known for its release with the IBM Personal Computer in 1981, and its subsequent release with the IBM 5170, where it was reconfigured with the AT protocol and some layout revisions.[2]

The capacitive design is widely considered superior to that of the later membrane design used on the Model M. 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'  ? Released in 1981 with the IBM PC 5150. Uses an XT connector. 83-key PC/XT keyboard
Model F 'AT'  ? Released in 1983 with the IBM PC 5170. 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. 84-key PC/AT keyboard
Model F '104-key terminal keyboard' 1387033 Terminal keyboard released for the IBM 5085 and IBM 3290.[3]
Model F '4704 62-key' 6019284 62-key keyboard released for the IBM 4704[4]
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,[5] these Model F keyboards featured fully white keys.

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 more complex to manufacture and service.[6] 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.[7]

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.[8]

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.[9]

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 ( Metal in the 4704 series ) Molded (unpainted) plastic
Internal stabilizer Only in early models No
Buckling spring implementation Capacitive plate Plastic membrane
Key rollover Unlimited 2-key rollover[10]
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]