17 Oct
2012
17 Oct
'12
2:43 a.m.
On Tue, 16 Oct 2012, David Riley wrote:
That's not really the problem. There are plenty
of open-collector
TTL chips; the ones that I've found come closest to what you'd want
(for QBUS, at least) are the 74AS640 and 74AS760. They're both
The '640 is tri-state. You mean e.g. the '642. Only problem: They are
almost unavailable and VERY pricy.
The "ideal" IC solution today would be the SN7438, still produced. It is
almost identical to the SN7439 that DEC used as bus drivers (equal to
N8881). Otherwise just use some SN7406 as drivers; they are adequate in
most Unibus applications today where you don't have bus lengths of several
meters.
See above. My major issue with them is that they pull
down too
fast (QBUS has a minimum of 5 ns).
Is that a real-world issue or just hypothetical? We have a custom
external semiconductor memory expansion for our 11/20 where they used
SN7401 as bus drivers!
Christian