9 Sep
2005
9 Sep
'05
5:37 p.m.
The mentioned 7480 is maybe hard to locate even as a
datasheet (good
idea to keep old TI TTL databook, IIRC
Why do you think I keep (and actively grab) all old databooks :-)
also at bitsavers), and even more difficult to locate
the chip itself,
but this is a function that would easily fit into a GAL.
A real problem are species what can't be simply replaced by some GAL,
namely open collector components or "extendable"
gates like the 7450. Replacing a single chip of such a kind usually
I hate to tell you this, but the 7480 will not fit into a GAL. The A* and
B* pins are effectively open-collector I/O with intenral (6k) pull-ups.
You can extend the input gating via that pin, or you can monitor the
output of the A1/A2 NAND gate there.
means replacing its environment as well, e.g. that
7450 would
then require to pull the extension gate (was it 7460, my memory is
failing) as well.
Great fun if it happens to be on another board (did this ever happen?)
Surely the question original replacement vs.
reengineered functionality
has some religious aspect, but if it is the question of whether
an old system should be left inoperable in a museum in contrast to
actually switching its blinkinligths I favor reengineering.
Me too. Computers that don't work are not interesting. That said, I am in
favour of doing as few mods as possible to the original machine. For
example, I'd fit a turned-pin socket in place of the original chip and
wire up a heeader to the replacement chip (maybe using stripboard or a
PCB to make a plug-in module). That way, if I ever get the real part, I
can go back to the original design.
-tony