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

[Tony Duell]

> 
> ard at p850ug1.demon.co.uk (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 
different...

> 
> > 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.

-tony