DEC bus transceivers

Guy Sotomayor Jr ggs at
Tue Oct 25 11:10:01 CDT 2016

> On Oct 25, 2016, at 8:38 AM, allison <ajp166 at> wrote:
> 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.

That’s why there’s the UNIBUS spec that DEC published.  In big systems, you must
follow the guidelines even with the proper transceivers.  Sometimes it requires that
things be split up and cable length added.

Also, I think in a previous email you mentioned that the UNIBUS is 240ohm.  It’s not.
It’s 120ohm.

> 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.

OK, it’s still a small system by UNIBUS standards.  ;-)  I can’t check either my 11/40
or 11/70 but (from memory) the configs are:
BA11F(1): CPU with all options (including FIS), 2 32KW core memory, additional backplanes to fill it up
BA11F(2): 2 32KW core memory, 1 RK11D (4 slot backplane), additional backplanes to fill it up
The “standard” Unibus devices as I recall are (there are more as most of the backplanes are full):
4 DL11s
At least one tape drive interface of some sort
BC11A-15 to connect the two BA11Fs together.
BA11F(1): CPU with all options, Hypercache (4MW option that looks like a 4MW cache to the CPU), 2 SMD controllers that plug into the slots for the massbus controllers (4 slots each).  2 Massbus controllers (4 slots each), 1 DL11
BA11K: RK11D 4 slot backplane, 2 RL11 controllers, TS11 controller, TU81 controller, 2 DZ11s, 3 DL11s, 1 ethernet controller
There’s more on this system, I just don’t recall it all at the moment.  Again, this system is *full*.  I’m at the point where if I want to add more stuff I need to add another BA11K.
BC11A-15 to connect the BA11F to the BA11K
> 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.

Yes, and they all used the proper UNIBUS transceivers and not TTL parts.

TTFN - Guy

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