|Re: Keyboard Technology|
Message #2 Posted by Bernhard on 11 Apr 2006, 6:10 p.m.,
in response to message #1 by John Limpert
Good suggestion. Interesting topic. Keyboards are the key to calculator success. From my reverse engineering:
Early Classics: gold galvanized metal strips (built as in the patent)
Late Classics: no gold, rest is the same
All Classics: keyboard can be disassembled with a screwdriver
HP21,...,27 metal strips. Keyboard can't be disassembled except
when heat-formed plastic domes are cut away
HP29C ? possibly (I guess !) the same sort of plastic snap
action contact foil as in early spices. My HP29C
feels like an early HP31E (stiffer keys, need more
force, longer key travel) and not like any of the
other HP2x. Its keyboard PCB also looks much
Early Spices: Metal strips replaced by plastic snap-action foil
(one piece for all keys). Contacts probably some
sort of metal-filled paint that is printed on the
Late Spices: The proven metal strips again.
All keyboard keys in all those models are hinged. They can't wobble,
greatly increasing life of the contact elements. A typical failure mode is a hairline crack in the metal strips just where they are
bent most when they snap. Those keys feel mushy but usually still work. The plastic key snap-action foil never lost the snap effect on any of my HPs, but some of those keys don't give any contact at all.
If the metal strip keyboards have bad contacts they usually can be cleaned and brought back to reliable operation.
TI calculators of the time had much thinner metal strips with a row of round discs. They were held in position by a self-adhesive plastic sheet. TI keys were not hinged and had a wobbly feel. Worse yet, the counter-contact was a round wire that ran down the keyboard column and was welded into a plastic keyboard backbone, giving a very tiny and unreliable contact area.
So far what I have found out. HP keyboards were the best. All other calculators were worse and their keyboards failed often, except for those who had tiny reed-switches, like the Olympia CD80, too expensive and without the tactile snap-feedback, but truly lasting forever - which HP keyboards didn't. It seems however that HP keyboards had just the right price, performance, including decades of daily use lifespan, and the right look and feel.