Author Topic: Compaq Portable Keyboard uses... 12-volts?  (Read 242 times)

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Offline katastic

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Compaq Portable Keyboard uses... 12-volts?
« on: June 21, 2021, 04:11:19 pm »
I've got a Compaq Portable. (I) the big suitcase one, also featured on EEVblog, IIRC. I'm looking into adapting this keyboard for normal PC use and I ran into something strange.

The keyboard uses (AFAIK) the standard AT keyboard protocol of the original IBM PC. With one caveat. Instead of sending 5-volts over the cable it sends... 12-volts over. At first I thought it was doing some sort of RS-232 or something instead of TTL, but no, after buzzing everything out it simply sends 12-volts from the motherboard/PSU, and then slaps on a 7805 immediately after which regulates to 4.88 volts. As long the incoming voltage is >6.8 volts, it regulates at 4.88v steady, all the way up to 15 volts my bench PSU tops out at.

Anyway, the question I have is... does anyone have any idea why someone would 1) go around spec and re-invent the wheel with their own powersupply? The onboard compaq PSU has plenty of 5-volt capacity, and the keyboard consumes maybe 20 milliamps.

Noise issues? Keyboard re-use with other models? (I don't think there's any. This is Compaqs first computer, IIRC.) I mean, if the power rail was too suspect to noise, then certainly the communication lines would also.

I can easily work around it. It's just odd and I can't come up with a plausible design reason for it.


Online james_s

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Re: Compaq Portable Keyboard uses... 12-volts?
« Reply #1 on: June 21, 2021, 06:30:39 pm »
I have an old dumb terminal that has a similar approach, I assume they did it to ensure that there is a stable 5V at the end of the thin conductors in the keyboard cable back when the internal logic in the keyboard would draw a significant amount of power.

I'm curious why you'd want to adapt the portable keyboard to work on something else though. An original Compaq Portable is a valuable collectible, the keyboard is pretty terrible as I recall from the one I had back in the 90s.

Offline Rasz

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Re: Compaq Portable Keyboard uses... 12-volts?
« Reply #2 on: June 22, 2021, 12:01:22 am »
Everything was overbuild back then. 12V means healthy safety margin.
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