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