> On Oct 25, 2016, at 4:31 PM, Guy Sotomayor Jr <[email protected]> wrote:
>
>
>> On Oct 25, 2016, at 1:09 PM, Paul Koning <[email protected]> wrote:
>>
>>
>>> On Oct 24, 2016, at 4:48 PM, Guy Sotomayor Jr <[email protected]> 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
