Do you think a command/data line is necessary? I thought about it (since I've been working with LCDs lately) but it seems that any given byte we send in either direction is going to be a part of a predefined packet of data that will be defined by the protocol. The first byte sent to any given slave will always be some kind of command and the rest of the packet will be supporting data for that command.

Or am I wrong?

What we probably ought to consider is either having a line (or using an existing line in an additional fashion) that would allow the central 8052 to send a "sync" command that could be sent before any packet to make sure that the slave is in-sync in receiving a new packet. This could be done just by the fact that the slave select line goes high.

Regards,
Craig Steiner