>At the time we didn't discuss how the master and slave
>synchronize (the master can't start transmitting until
>the slave reads the pulled up/down state).  What is the
>best way to do this?  The original poster was trying to
>reduce the number of I/O pins used and wanted the
>hardware and firmware to be as symmetric as possible.
>
>The "cheapest" way to synchronize that I could see is to
>designate one pin on each PIC (output on slave, input on
>master) that means "slave ready to receive."

Hmm, this may not work for this situation if both PICs are on the same power
supply, unless you slug the MCLR line of one with a larger capacitor than
the other.

However a scheme I have seen used on a multidrop mini computer system went
as follows: -

machine is powered up. It then proceeds to listen on the serial line for a
period of time to determine if there is already a master device. If there is
activity then it assumes the slave role, else assumes master role after time
out of the listen period.

Note that this involved two machines which ran identical software, and the
time it took from power on of one machine to power on of the second machine
gave enough delay for the first machine to assume master state. In this case
they kept each other in sync by the master first talking to the slave with a
message "I am going to call device x for data" to which the slave would
respond with an affirmative message, and then both would listen to the
answer from device x, and record it.

You may be able to implement something similar in your code, but personally
I would seriously look at having a conditional assembly to set one copy as
master, and the other as slave, on otherwise identical source code. It may
even be a flag in eeprom that is programmed later, or a single location in
rom that gets an updated program later.

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