I think (as I stated in previous post) that you should consider the centronics I/f (or its successor the IEEE 1284). It provides in either simple unidirectional, or bidirectional communication between two devices.(depending on the mode)

more info here: http://www.fapo.com/1284int.htm

and of course google and the IEEE.

advantage:
- it is standardised and used a lot. Your experience gained and your software developped will be more "(re-)useable". The software developped can be saved as a library for future use... (connecting to PC or printer...)

As for the protocol layer used on top of this transmission, you could stay with your protocol structure, ie first byte is command, followed by data, followed by crc...
Just make sure you have a decent description before implementing the protocol.

regards,

PAtrick