Ethan Dicks sez...
>
> --- Jim Davis <jpdavis_at_gorge.net> wrote:
> > Ethan Dicks wrote:
> > >That's been my experience, too. I have replaced as many as 8 7474s
> > >at a go to get an -8/L back on its feet. I don't recall replacing
> > >any other chip, as a matter of fact.
> > >
> > >-ethan
> > >
> > 7474's Dead d flops? whats the story on that?
> > Jim
>
> Dunno. I can say categorically that they were failed chips. They
> did not pass muster in a handheld TTL tester, and when replaced,
> the PDP-8/L worked.
>
> There was a mention on the list of a batch of 7440s that failed in
> another M-series machine (might or might not have been an -8/L)
> with the same date code.
>
> Not sure why 7474s from the mid-to-late 1960s would be more fragile
> than, say, 7400s. Perhaps the flipping and the flopping wears them
> out faster than anding and oring? :-)
:-)
In my case, the failure modes were as follows:
2 chips failed with: grounding the PRESET line caused the Q
to go high as expected, but when PRESET was left floating Q
went low. [DEC leaves the PRESET and CLEAR lines floating on
various modules, so I consider this a fair test].
1 chip failed with insufficient +5 on the Q (or bar-Q, forget
right at this instant) output. The measured output was something
like 1.6V.
Like Ethan sez, when these chips were replaced, things got better.
I think the "reason" might be that these chips were pushing the edge
of semiconductor manufacturing technology -- they are certainly one of
the more complicated chips from the early 74xx (all 14-pin only :-))
era. That's my theory...
Cheers,
-RK
--
Robert Krten, PARSE Software Devices +1 613 599 8316.
Realtime Systems Architecture, Consulting and Training at www.parse.com
Looking for Digital Equipment Corp. PDP-1 through PDP-15 minicomputers!
Received on Sun Jul 13 2003 - 18:50:01 BST