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.[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.
Modern re-creation
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
Design
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
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.[
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 (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
|
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
|
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