PDP-8m Console Switch Problems - fixed!

[Don North]

Brent Hilpert wrote:
> Observations:
>  - behaviour may differ between IC units even within valid Vcc range,
>  - a given IC unit may change its behaviour within or near valid Vcc range,
>  -  175 class may pass at lower  Vcc and fail at higher Vcc,
>  - S175 class may pass at higher Vcc and fail at lower  Vcc.
>
> Interesting the way 175 and S175 devices differ in their response to Vcc change.
>
> The group of TI S175s from 1979 did seem to show reliable behaviour over the
> entire range.
>
> One way or the other, the DEC front panel is relying on unspecified behaviour
> of the device. I wonder if the designers were just relying on old habits of
> setting flip-flops via collector triggerring from the discrete days, and just
> got lucky that it worked.
>
> (Another observation was that the clock input *must be held low* for any
> of the devices to work as desired.)

Very interesting. I hadn't run any tests varying the VCC voltage in my 
tests.
I'll have to try doing that on the set of parts I've already tested.

I also went over to ACE today in San Jose and got half a dozen samples 
each of
TI, NSC, Fairchild, Signetics 74175 and 74S175 from the late 70s/early 
80s to test.

But based on Brent's data alone I may go back and look for TI S175s from 
the late 70s.

Don