> 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.
>
> You have with just two fixed lines bidirectional tranfer with handshaking
> built in!
>
> This is how the parallax programmer comunicates the the PC with just two
> fixed direction lines. Its  brilliant!

For this protocol to work, however, it is necessary that the device
responding to the clock do so within a known, finite, amount of time
and this time limits the speed of communications (e.g. if the slave
may take up to 1ms to respond to the clock pulse, then the master
must wait 1ms after every bit to ensure that it's reading the correct
data).  There are some cases in which this requirement is acceptable--
often a micro will have nothing better to do than wait for clock signals
from a PC.  But if both devices may be genuinely busy, I don't think
any approach without two bidirectional wires or latching hardware will
work.