DEC bus transceivers

Guy Sotomayor Jr ggs at
Tue Oct 25 09:02:35 CDT 2016

> On Oct 24, 2016, at 11:35 PM, ben <bfranchuk at> 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

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