17 Jan
2008
17 Jan
'08
10:31 p.m.
"Roy J. Tellason" wrote:
Look in the archives around Aug/Sep 2006 for subject with
"PDP-8m Console Switch Problems"
The gist was that the front panel was using the '175 in a non-specified way
(FF collector triggerring) and a replacement device did not function in the same
way. It turned out that the behaviour could vary with small changes in Vcc amongst
devices from different manufacturers and different classes (74,74S,74LS).
.. just an instance of device behaviour being sensitive to small Vcc variation.
On the other hand, the early HP2100-series processors use some 74xx TTL
operating at 4.5V on IO/interface boards, which is outside the Vcc spec.
I've been a little surprised at HP for doing that.
(Yes .. (my comment was intended as a bit of a joke))
---
I've never seen the documentation for DEC's early discrete flip-chips
(bitsavers?) but doesn't it include full specs for things like loading rules
and fan-in-out, seeing as the flip-chips were sold as a 'logic family' to build
one's own system from.
Anyone know offhand how many transistors were in the first PDP-8? It's parallel
and core-mem and thus outside the OP's objective but could be interesting for a
ballpark comparison.
(I know somebody on the list has a site listing all the -8 versions) but was
the bit-serial version of the -8 done in discrete or IC? If IC, could be
interesting to translate it back to discrete and estimate the transistor count.
A year or two
ago there was a discussion on the list where we were testing
a variety of 74x175's (albeit configured in an odd manner) for some PDP-8
front panel and were observing changing and idiosyncratic behaviour over a
very narrow operating voltage range.
Do you recall offhand what sorts of changes in behavior was being discussed? I hope
you're not looking to mimic TTL - you'll have trouble sourcing
multi-emitter transistors in discrete form.
I seem to recall some wikipedia page referring to those as if they ever were
actually available. Not that I could say with any certainty that they
weren't, and if anybody knows otherwise please feel free to point me toward
info, but I've never seen them, or heard of them as discrete devices at
all.