DEC bus transceivers

Guy Sotomayor Jr ggs at
Mon Oct 24 13:13:23 CDT 2016

> On Oct 24, 2016, at 10:37 AM, allison <ajp166 at> wrote:
> On 10/23/16 2:59 PM, Al Kossow wrote:
>> On 10/23/16 11:50 AM, shadoooo wrote:
>>> The problem is that there aren't open drain bus transceivers, but the
>>> problem could be solved simply using input-only and output-only components,
>>> connecting two in parallel but opposite direction on bidirectional pins.
>> The reason for using the old parts is the logic thresholds are unique to
>> the Unibus to handle worst-case bus loading and the termination voltage they
>> used.
> The voltages are based on TTL levels.  What are the unique voltages?
> The key was limited leakage current and input current to not load the bus by inserting or removing
> current from a node (there is a specified maximum in per node and total nodes).  That cover input
> to card devices and bus driver leakage.
> Logic low voltage is typical of TTL and the driver device has to sink that current and meet that value.
> Logic High was set by the terminator devices at 3.36V but the threshold is lower based on the bus
> receivers.
> By late 1970 it was an easy spec to meet,  When first used (pdp8e) it was new and the ICs
> were not so great with leakage current and output device saturation current.
> Every time this comes up the world is supposed to stop if not met. The LSI-11 bus (qbus)
> was actually harder as it was 120 ohm terminated and HeathKit did it with common TTL
> and the CPU was DEC standard LSI-11 and it worked out to 18 slot backplanes.

The biggest concern is when interfacing to UNIBUS.  In the PDP-11 UNIBUS Design Description
document on Bitsavers, page 4-1 indicates what the Unibus interface chips are and what are *not*
recommended (8640, 8641 and 8881 are the only ones recommended).

There are a number of rules that must be adhered to when building out a Unibus system.  These
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.

A single Unibus can be divided into multiple segments.  Each segment must adhere to the above
rules, so you can see that a Unibus can be quite large.

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.

My point here is that the Unibus has a very different electrical environment than Q-bus or Omnibus
and what may work for them will probably have troubles on a Unibus.

TTFN - Guy

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