Re: Slot 'extender' dev cable

["Michael J. Mahon" <mjmahon@aol.com>]

mdj wrote:
> On Jan 31, 7:25 pm, "Michael J. Mahon" <mjma...@aol.com> wrote:
>
>
> > Unfortunately, the answer is "It depends."
>
>
> There goes my hope that you were going to say "the ESR and capacitance
> of IDC ribbon is sufficiently low that all but the most aggressive
> card designs should work fine" ;-)

;-)

> > If you have few cards plugged into your development machine, and
> > those that are present put only a modest load on the address and
> > data busses, a 2- to 3-foot extension cable is probably workable.
>
>
> Actually the dev machine is as unexpanded as I can get it - just a
> Disk II card and an old Atlas 40 tracker, plus one of the miniaturised
> late model extended 80 column cards.
>
> I am somewhat paranoid about plugging in stuff I've designed with
> harder-to-source peripherals until I've proven my design operates as
> expected.

A very sensible approach!

> > The danger is that it will inevitably alter the risetimes and
> > falltimes, and add several nanoseconds of delay.  You may wind up
> > with your design being dependent on these timing modifications,
> > and being marginal when directly inserted into an Apple II.
>
>
> Hopefully, several nanoseconds won't bother this design, which is
> effectively an asynchronous, buffered two way communications setup -
> so there's really only a couple of transceivers and latches, and a
> tiny bit of address decode logic.

Odds are that it will work fine.  Just do it!

> > Of course, if your timing design is conservative, that wouldn't
> > be a problem--but it's sometimes hard to know.  ;-(
>
>
> Well, when the CRO shows up I'll see if I can measure the propagation
> delay of the cable. I'll let everyone know.

The cable delay will be just a little more than 7ns per foot,
the speed of light on wires being what it is.  ;-)  The effect
on rise and fall times will be small but perceptible for 2-3 feet
of good quality cable.

I'd recommend liberal use of 0.1uF decoupling capacitors on the
card, since the cable represents a modest amount of inductance.

And the safest thing to do is examine all your critical timings
both with the cable (during development) and witout the cable
(when you think you're done).

-michael

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