Question here is: How do you know a transmission has started? How do you know what you "read" on the bus is not the bus being idle, but actually a 9bit word being present?

How does the receiver know that the data is on the bus and valid?

On the other hand, if the bus is bidirectional, how will you prevent contention (ie two 8051 putting data on the bus at the same time...)

I think you do not necessarily need an inband handshaking (ACK/NACK), I think that hardware handshaking is probably easier to implement (ie use a signal for data-valid and eventually a data-direction signal..) Check the centronics printer i/f and successors as th latter should provide in bi-directionality.


Just some thoughts... If you think you have the problem sorted, don't let my worries stop you from implementing it.

regards
Patrick