HP Forums
HP48 Series Keyboard Design - Printable Version

+- HP Forums (https://www.hpmuseum.org/forum)
+-- Forum: HP Calculators (and very old HP Computers) (/forum-3.html)
+--- Forum: General Forum (/forum-4.html)
+--- Thread: HP48 Series Keyboard Design (/thread-6653.html)

HP48 Series Keyboard Design - TomC - 08-04-2016 03:49 PM

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,

RE: HP48 Series Keyboard Design - cyrille de brébisson - 08-05-2016 06:26 AM


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.


RE: HP48 Series Keyboard Design - TomC - 08-05-2016 09:19 AM

Cyrille, thank you!