Re: Slot 'extender' dev cable

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

mdj wrote:
> Hello,
>
> One for the electronics guru's...
>
> I've recently been hacking together a new Apple II card design, and
> have recommissioned my 'dev' cable, which is a couple of 16 way
> ribbons running from a cut down card to a pair of IDC DIL connectors
> allowing me to easily export A0-7, the data bus, plus appropriate
> power and signaling to a breadboard.
>
> Now, it has occurred to me that the data bus might struggle a bit with
> this setup, but unfortunately until my new CRO turns up (old one
> recently bought it) I have no easy way to measure the net effect of
> the cabling.
>
> Having said that, it seems to work fine (the cables are about 1.5ft)
> and D0-7 feed directly into a '245 so there's probably nothing to
> worry about, but I am beginning to wish the cabling was longer, and
> thought I would ask the experts what the reasonable limits of such
> slot extension might be.

Unfortunately, the answer is "It depends."

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.

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.

Of course, if your timing design is conservative, that wouldn't
be a problem--but it's sometimes hard to know.  ;-(

-michael

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