Model F keyboard

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 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 5170, 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. 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

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

Name	PN	Description
Model F 'XT'	?	Released in 1981 with the IBM PC 5150. Uses an XT connector.
Model F 'AT'	?	Released in 1983 with the IBM PC 5170. Uses an AT connector.
Model F '122-key terminal keyboard'	611034x	Terminal keyboard released for the IBM PC 3270. Uses an IBM proprietary terminal connector.
Model F '104-key terminal keyboard'	1387033	Terminal keyboard released for the IBM 5085 and IBM 3290.^[4]
Model F '4704 62-key'	6019284	62-key keyboard released for the IBM 4704^[5]
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,^[6] these Model F keyboards featured fully white keys.

Design

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.^[7] 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.^[8]

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.^[9]

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

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.^[10]

Comparison with Model M

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
Buckling spring implementation	Capacitive plate	Plastic membrane
Key rollover	Unlimited	2-key rollover^[11]
Assembly method	Reusable metal tabs	Single-use rivets
Spring barrels	Individually inserted in a metal plane	Single plastic mold with predefined barrels

References

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