PDP-8m Console Switch Problems - fixed!
ak6dn at mindspring.com
Fri Sep 8 19:04:55 CDT 2006
Brent Hilpert wrote:
> - 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
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
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.
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