> >Here is my question:  How do I write the code so that
> >the Slave knows which byte is the "command" byte and
> >which byte is a "data" byte?

I have been using SPI to send a packet from one controller to the PIC
(Kronos Robotics Dios chip) using the following protocol:

254       --    Start-of-packet signal
motor     --    0-6, selects motor to control
speed     --    0-15, selects speed
direction --    0-2 (forward, backward, braking)
0         --    sent so that the controller can read the checksum

Basically I create a small state-machine, which an integer listing
what state I should be in. If I get a bad value at any time, I abort
the transaction and the sender doesn't get a valid checksum, so the
sender resends the entire packet.

I added the checksum because otherwise is is difficult to know if the
reciever has actually recieved the packet.

I hope this helps.
--
D. Jay Newman           !  I can bring whole cities to ruin,
jay@sprucegrove.com     !    And still have time to get a soft-shoe in...
http://enerd.ws/        !      -- from Buffy, "Once More with Feeling"

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