> On Oct 25, 2016, at 1:09 PM, Paul Koning <[email protected]> wrote: > > >> On Oct 24, 2016, at 4:48 PM, Guy Sotomayor Jr <[email protected]> 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
