17 Oct
2012
17 Oct
'12
5:18 a.m.
On Oct 17, 2012, at 5:43, Christian Corti <cc at informatik.uni-stuttgart.de>
wrote:
I'm sure it works well enough in practice, especially
on small systems. I think the DEC bus specs are probably
excessive until you get to a many-meter backplane with a
bunch of extensions which add impedance discontinuities;
I imagine then you start to see why their thresholds
are set where they are.
A first pass using commodity chips that "just work" might
not be a bad idea just to get the ball rolling; more
compliant solutions can be devised later, and a
7438 solution isn't going to blow anything up.
- Dave
On Tue, 16 Oct 2012, David Riley wrote:
Uh, yes, that's what I meant. Sorry! In any case, they're
usually fairly available at Mouser, but yes, they're somewhat
pricy.
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.
Those would probably make OK drivers, though I'd
personally prefer something that came closer to the Iol of
70 mA (again, I'm thinking more about QBUS, which
specifies the maximum output voltage when sinking
70 mA).
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!