DEC bus transceivers

[Pete Lancashire]

Guy S.


Thanks saved me the time to say exactly what you said. For all those that
thing designing a driver is a simple thing to do, make your self a
simulated Unibus that is 50 feet long, add around 30 'stubs' load it down
to the max and show me your 'easy to build' drivers signal quality.

On Mon, Oct 24, 2016 at 11:13 AM, Guy Sotomayor Jr <ggs at shiresoft.com>
wrote:

>
> > On Oct 24, 2016, at 10:37 AM, allison <ajp166 at verizon.net> 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
> 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.
>
> 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
>
>
>