> On Oct 24, 2016, at 11:35 PM, ben <[email protected]> wrote: > > On 10/24/2016 2:18 PM, David Bridgham wrote: >> On 10/24/2016 01:37 PM, allison wrote: >> >>> The voltages are based on TTL levels. What are the unique voltages? >> >> The QBUS spec from the 1979 Bus Handbook (the Unibus levels are the same): >> >> Input low voltage (maximum): 1.3 V >> Input high voltage (minimum): 1.7 V >> >> And from the TI datasheet for the 74LS74: >> >> Vil - low-level input voltage 0.8 V (maximum) >> Vih - high-level input voltage 2 V (minimum) >> >> So no, the DEC bus voltage levels are not TTL levels. Yeah, TTL might >> work on a smaller system but you can see that if you push it out to its >> limits, TTL could start getting flaky. That's the kind of bug I'm happy >> to have DEC's engineers figure out and not have to track down myself. >> > But who has the big systems now days? The days of 4K core is long gone. > Use TTL and try to keep the systems small.
I just posted what one of my systems is (11/40). It has 128KW of core which takes 4 9-slot backplanes. I have a fair amount of I/O on that system so I have 2 BA11F chassis *full* of backplanes (each BA11F holds 5 9 slot backplanes). My other large system is an 11/70 with a BA11K as an expansion box. The whole point of Unibus was to allow for large configurations. And I can guarantee you that TTL will *not* work in those systems. If you want to build boards that will work in a small subset of systems that’s find…but don’t advertise it as Unibus compatible. I test the boards I produce in all of my systems (11/20, 11/34, 11/40 and 11/70) and they all use DEC bus interface chips. TTFN - Guy
