DEC bus transceivers

[Paul Koning]

> On Oct 25, 2016, at 4:31 PM, Guy Sotomayor Jr <ggs at shiresoft.com> wrote:
> 
> 
>> On Oct 25, 2016, at 1:09 PM, Paul Koning <paulkoning at comcast.net> wrote:
>> 
>> 
>>> On Oct 24, 2016, at 4:48 PM, Guy Sotomayor Jr <ggs at shiresoft.com> wrote:
>>> 
>>> 
>>>> ...
>>>> Where do you see the 25 ns spec?  I didn't see it (admittedly in a quick scan).
>>> 
>>> 5.2.7.  It’s discussing the AC loading as a percentage of the risetime (25ns) to allow for the
>>> reflections.
>> 
>> That seems more like a "for illustration" than an actual specification. 
> 
> Maybe, but when you read section 5.2.7 of the PDP-11 Unibus specification:
> Nine lumped ac loads reflect 20 precent, and 20 lumped ac loads reflect 40 percent of
> a 25 ns risetime step.

That's my point.  It says "a 25 ns risetime step".  Not "THE 25 ns risetime step".  Nor "at the minimum 25 ns risetime step".  So it seems to be illustrative: if in your particular config the risetime happens to be that, the example holds.

The obvious question to ask is what the bus capacitance is, between the module contributions, and the distributed capacitance of the cable.  That  would explain (in part) the risetime.  The other part would be the inherent risetime of the TTL components of the era, which are somewhere in that range judging by some of the old datasheets I've just been looking at.

	paul