DEC bus transceivers

allison ajp166 at
Wed Oct 26 05:16:02 CDT 2016

On 10/25/2016 04:24 PM, Guy Sotomayor Jr wrote:
>> On Oct 25, 2016, at 12:40 PM, allison <ajp166 at> wrote:
>>> Also, I think in a previous email you mentioned that the UNIBUS is 240ohm.  It’s not.
>>> It’s 120ohm.
>> My book says no. Qbus is for sure 120.
> Section 5.2.5 of the PDP-11 UNIBUS spec:
> A Unibus segment must always have a Unibus terminator at each end of its
> 120-ohm transmission path.
> So, I don’t know how you get a value of anything other than 120-ohms given that
> statement.
Its not.  It may be close but....  What your reading is the terminator
value (R1+R2)/R1*R2=120.
thats a terminator and its also limited in the range of values because
the DEC part can only
sink so much and also the terminator has to source current as the driver
can't.   Likely
the line impedance can be in the range of 120 ohms, maybe.  I"ll have to
drag out a
Qbus back-plane and measure it.  I don't have any Unibus.

As far as the chips themselves.  They do NOT match the line impedance as
they have
a active low value in the 10 ohm range and when in the high state they
are just an open
or somewhere in the effectively infinite range certainly more than
several thousand ohms.
That's a terrible mismatch for transmission lines, aka bus.  The
combination of wacky
source impedances and near open load impedance (input current varies
with source
voltage Vih and Vil) plus parallel capacitance from package and traces
on board means
that as a load its a horrid mismatch as well. With all that reflections
(ringing) are to be
expected and the only thing that can help that is the terminator even
then only to a point.
In short most TTL are no better and those designed to do bus interface
are about the same.
Also CMOS would be far worse as a receiver and some CMOS makes a better
but they are incompatible with Unibus/Qbus as they actively source current.

The only common tech that does line matching especially from that era is
ECL, or current
mode logic.

This is why faster buses  are so difficult to make right.  And if
possible they are to be
avoided or made small as possible.  Been there and done that,, mixed
signal design
for the last 40 years.


> TTFN - Guy

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