HP48 Series Keyboard Design

[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