26 Sep
1997
26 Sep
'97
2:17 p.m.
Fortunately they were not commonly used in old
computers either. I think Sun
used them on some Sun 3 boards (I seem to remember them when I was
looking inside my 3/260), but that's about the commonest place.
I did not see any on a 3/280 board, but then it could have been done by a
different engineering team.
Oh, of course. Second-hand chips are more of a risk
than second-hand
machines - the machines (and peripherals) can almost always be repaired
or used as a source of spares, but dead chips are useless.
Second hand chips better be cheap, especially if they have "reject"
stamped all over (yes, I have seen some people trying to sell stuff like
that!). A guy I knew back in Illinois would go and raid the
Rockwell/Collins surplus outlet and sell off the chips quite cheap ($1 for
a tube of 128Kx8 RadHard EEPROMS - that tube cost Rockwell $24,000!). He
would often have very bizarre chips (10G series GaAs, for example), many
mil-spec. Many had dots on them. After a bit of research, it turned out
that red dots meant dead chips, yellow dots meant cosmeticly flawed but
electically OK chips (Rockwell is as picky as IBM), and no dot meant new
surplus.
Be careful if you're buying tubes of PALs or OTP
PROMs. Often they have
been programmed with code that's totally useless to you and of course
they can't be erased. EPROMs are less of a risk there.
I never buy PALs unless I know the guy that is selling them.
I've never seen 24 pin ones. There were 16 pin
ones pinned out like a
4116 (but with the A7 line N/C), with multiplexed address lines and 22
pin ones with non-multiplexed address lines. I have a row of the latter
in an HP1350 graphics unit. But almost all the ones you find are one or
other of those.
Two pins too many - I meant 22 pins, on the odd 0.400 inch packages.
William Donzelli
william(a)ans.net