HP 8660C Keypad

[HaaTa]

Received something interesting today.



It's a piece of an HP signal generator (doesn't work standalone). And it has a very unique keypad.
The switching mechanism is not Hall Effect, Contact Based, Capactive, or Optical.
It's been described as a "Pulse Transformer Switch" by a friend of mine, which is apparently common in basic key switch circuits. But apparently extremely rare for keyboard switches, because I've never seen or even heard of one till now!

I actually have full schematics of the beast, so I should be able to make it work eventually (the keypad that is).
Anyways, time for pics!




There's the transformer for the keypad.


Gah my eyes, burning, too much gold...


Microswitches by Microswitch, who'd a thunk it
















Yes, those are plastic screws...

[Half-Saint]

Look at the thickness of those caps! With years they're getting thinner and thinner..... cheapskates!

[Charlie_Brown_MX]

Nice find. When was it made? My understanding is that HP stopped routinely making truly great hardware in the late ’80s, with some exceptions (HP 48GX, for example). Their Made in USA Voyager calculators (HP-1xC series, early ’80s) are absolutely fantastic; I have two and would recommend them to anyone who wants a good calculator. They use RPN entry, though, so there’s a bit of a learning curve but it’s worth it.

[Icarium]

Sweet, have you found a description of how it works, yet?

[486]

Nice Haata. I love those early keycaps, they look so retro. I have three later HP keyboards for use with terminals and majority of them are leaf spring but they have neat triple shots.

[Trent]

That is indeed a nice find. Do we know anything about the switch mechanism?

[HaaTa]

Looking at the dates on the ICs, looks like one piece was built in 1980/81 while the rest of it seems to be from around 1986 (there were multiple versions of the 8660; A,B,C,D with C being the most popular/longest running).
Though the initial versions were from the 70's, but not all of them had the keypad apparently. And the keypad on 8660C (this one) is different from the keypad on 8660D.

Here's a link to the schematics of the 8660B, which seems to have the same keypad.
https://docs.google.com/open?id=0B4Wyz4 ... nB2WGFqck0

Page 44 (8-110) has a description on how the switch mechanism works.

tl;dr
Matrix is scanned, by powering one side, and looking on the other.
When a key is "pressed" the spring gets closer to the spiral magnetic pads, it causes an "inductive short" which results in a keypress detection (1/0) on the line.

Hmm, I guess I'll coin this as an "Inductive keyswitch type", sub-type Pulse Transformer switch (cause it sounds cooler ).

[HaaTa]

Nice Haata. I love those early keycaps, they look so retro. I have three later HP keyboards for use with terminals and majority of them are leaf spring but they have neat triple shots.

Leaf Spring?
Pics! Where are they.

[486]

Sorry about the pics. I didn't want to have to dissasseble.

[HaaTa]

HOLY CRAP!

I didn't know HP used Fujitsu Leaf Springs. I was under the impression that they were all domes.
Must get one!

Any pics of the rear label/model number?

[webwit]

they have neat triple shots.

More pictures!