DEC bus transceivers

Guy Sotomayor Jr ggs at
Tue Oct 25 15:31:36 CDT 2016

> On Oct 25, 2016, at 1:09 PM, Paul Koning <paulkoning at> wrote:
>> 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. 

Maybe, but when you read section 5.2.7 of the PDP-11 Unibus specification:
Nine lumped ac loads reflect 20 precent, and 20 lumped ac loads reflect 40 percent of
a 25 ns risetime step.
>> 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.

My concern (and I get that the supply of DEC transceivers is limited) is that someone will build some thing and says it works in Unibus machines and they’ve only tested it in a relatively small system (one BA11K box for example) and it will fail miserably (or worse only fail intermittently) in larger systems.

TTFN - Guy

More information about the cctalk mailing list