DEC bus transceivers

Ethan Dicks ethan.dicks at
Mon Oct 24 17:30:41 CDT 2016

On Mon, Oct 24, 2016 at 4:35 PM, Guy Sotomayor Jr <ggs at> wrote:
> OK, I guess my last email didn’t make it.  It appears to me that the rise time is set at 25ns.
> You need to look at the PDP-11 UNIBUS Design Description document on Bitsavers.  Firstly,
> in section 4-1, it specifies which chips to use and recommends not using a whole list of other
> chips.  The only recommended chips are: 8640, 8641 and 8881.

We used 8641s on our Unibus COMBOARDs (which weren't built cheaply -
$2500 MSRP in 1984).  We also used a pair of DEC DC013s for "Unibus
Interrupt Logic" (and they were somewhat expensive, IIRC, but we used
them to minimize compatibility problems.  Customer confidence was
worth a couple hundred bucks in parts and engineering overhead).  We
likewise went with DEC chips for our Qbus product - we bought the DEC
Chipkits and used about 75% of them - the DC005s for sure, and I think
the DC010s, but didn't use the DC004s, IIRC.  Either way, everything
touching the bus went through a DEC chip.

> There are a number of rules that must be adhered to when building out a Unibus system.  These
> include:
> Maximum cable length must be < 50’
> Maximum DC loading < 20
> Maximum lumped loading < 20
> There are rules where cable lengths must be *increased* to avoid reflections.
> For example, my PDP-11/40 resides in 2 BA11-F boxes (23” tall) and are fully populated with
> Unibus backplanes (5 9 slot backplanes each) with a BA11-15 (15’ cable) connecting the two.

We ran machines in the 80s that were not quite as extensive, but one I
recall well was our primary 11/750 with the internal DD11DK full of
cards (UDA50, and a few other things), then a 25' BC11 cable to a
single BA11-K with 3X DD11DK that were full or nearly full - mostly in
that box were the Emulex CS21F 16-port serial cards (at least 5), any
Unibus tape controllers, up to 6 of our COMBOARDs, at least one DMR11,
and a handful of quad SPCs.  It all seemed to play nicely, but we
weren't running close to maxed out.  Most of our other machines had
1-2 DD11DK outside of the CPU backplane but not nearly as many cards
or bus traffic.  We never had any Ethernet hardware, so all of our
sessions were handled via local serial ports.  Lots of serial ports.


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