DEC bus transceivers

Paul Koning paulkoning at
Tue Oct 25 15:09:26 CDT 2016

> On Oct 24, 2016, at 4:48 PM, Guy Sotomayor Jr <ggs at> wrote:
>> ...
>> Where do you see the 25 ns spec?  I didn't see it (admittedly in a quick scan).
> 5.2.7.  It’s discussing the AC loading as a percentage of the risetime (25ns) to allow for the
> reflections.

That seems more like a "for illustration" than an actual specification. 

> Yes, all I’m saying is that folks have been looking at OMNIBUS and QBUS and those are
> much simpler electrical environments than UNIBUS.  You really need to pay attention to
> the fact that UNIBUS is really a set of transmission lines so in addition to critical levels
> and currents you need to worry about the transmission line effects (ie the AC components).

Sure.  But whether you look at it as a transmission line or not, in the final analysis there should be a small set of receiver and driver specs that matter.  In theory, they should be the ones listed in the DEC documents, neither more nor less.  In practice, there might be unspecified ones, such as max slew rate.  If so, that's easy enough to handle by introducing a suitable RC at the driver base (or gate).

As for the receiver, it seems that a TI 75140 (adjustable threshold line receiver) might do the job.


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