HP48 Series Keyboard Design

[TomC]

As the issue of HP48 series keyboard failures continue to arise - and I'm sure it will continue with time - I've been thinking about a more robust replacement.

As I recall, the keyboard interface is designed such that there is a significant level of resistance from each one of the keypad locations.

Is the amount of resistance for each of these keypad locations documented anywhere?

Thank you,
TomC

[cyrille de brébisson]

Hello,

From memory, the keyboard lines are multiplexed with the memory lines and require a 4K resistance.
In the 48 keyboard design, this 4K is obtained by using carbon conductive conductors on the keyboard flex 'PCB' which have an inherent resistance. These conductors are then tuned (by making them all approximately the same length) to this 4K resistance.

As I said, from memory! so it might not be 100% accurate.

Cyrille

[TomC]

Cyrille, thank you!