Gaps in the collection (was Re: rarest computers. )

From: Tony Duell <>
Date: Mon Aug 9 17:02:00 2004

> (Tony Duell) wrote:
> > Talking of HP using 555 timers, they used one to drive the power-on light
> > in the HP86 (and I assuem the HP97). [...]
> (nitpicking here, you are talking about the 87)

Yes, of course I am. The 97 is a Topcat calculator and is totally

> > So the LED is dark if the machine is off, on steadily if the machine is
> > on and no error (555 is held reset, so the output is low all the time),
> > and blinks (controlled by the 555) if there's an error.
> Actually the LED blinks when the machine is busy (e.g. running a program).

Drat!. That means I have to make a correction to the signal name on my
hand-drawn HP86B schemtic. Oh well... Better to make the correction than
have confusion later.

> This is very neat as with the HP85 you sometimes do not know whether the
> program has finished or is still running.

On the 85, the LED just runs off one of the supply lines via a resistor.

Received on Mon Aug 09 2004 - 17:02:00 BST

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