DEC bus transceivers

allison ajp166 at
Tue Oct 25 10:38:02 CDT 2016

On 10/25/16 10:02 AM, Guy Sotomayor Jr wrote:
>> 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.

Maybe if care is not taken.   Again there are TTL part and those 
designed for bus use.
Bad choice, bad results.   I've seen a few like that that broke even 
with DEC parts.

Then again My 11/73 is BA11S and Ba11N both with 18 slot 4mb ram (two 
cards), DELQA,
two RQDX3 (one for RX floppies, second for RD disks), RLV21 (in box2), 
CMS scsi,
  RXV21V-RX02, DZH11V and Parallel IO (ttl) in box 2 and Gigilo card 
(DEC sound
card for Qbus) in Box 1.   Likely a few IO I forgot.

By all DEC standards its unsupported config.  Bus signals still look good.

I've seen my share of BIG VAXen with many unibus crates too.

old millrat
> 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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