[snip]
>Two pics or any other devices, with one input and one output each. they
>don't ever change (great for the PC printer port). To communicate one drops
>its output the other then provides an ack etc.......  and then provides the
>clock for the sender. Work out the details yourself  but I've given you the
>idea.
[snip]

Hmm.  This one's fine provided that the sender can keep up with the clock
provided by the receiver.  John Payson's go can cope with either end being
busy at any time for as long as required.

I think I've figured out how to apply it to many devices using the third
line, but perhaps John would be kind enough to post that bit too?

Dave