DEC bus transceivers

Guy Sotomayor Jr ggs at
Mon Oct 24 15:35:02 CDT 2016

> On Oct 24, 2016, at 1:30 PM, Paul Koning <paulkoning at> wrote:
>> On Oct 24, 2016, at 3:55 PM, David Bridgham <dab at> wrote:
>> On 10/24/2016 12:01 PM, Paul Koning wrote:
>>> I don't know about the receiver part, but I'd expect that the drivers could very easily be done with a simple transistor circuit.
>> Agreed.  However ...
>>> As for slew rates, unless you have antique transistors, that's not going to be an issue given that you meet the current sink spec; the slew rate of an OC circuit is determined by the system capacitance and the sink current of the driver.
>> I think you read this part backwards.  The slew rate requirement is not
>> a minimum slew rate but a maximum one.  That is, any modern transistor
>> (probably ancient ones too) will be way too fast.  You have to do
>> something to slow it down.  Still, I think this one is easily met as
>> it's just a series resistor on the gate of the driver MOSFET working
>> against the gate capacitance.  Some FPGAs have current limiting on their
>> output which may obviate the need for the resistor even.
> I don't see any max slew rate spec in the driver specs in the peripherals handbook.

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.

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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