> What about using an external I2C interface with a UART on it?
> 
> More parts, but might be easier than rolling the code too?

I should be OK with altering the code I've got thanks. With the F88s
they were on a simple I2C buss. Simple in that the master sends out
1pps to tick all the slaves' clocks. Each slave is assigned a window of
10ms within that second according to its address, so there'll be no
buss collisions. Also, as confirmation the other way, the master knows
by the time from the last tick pulse which slave is active on the buss.
So as long as I keep that timing system in place, I have fairly free
reign over what comms I design. I think probably the simplest will
be write a second UART for the 4550 and send to the UARTs on
the 1320s. That keeps it at 2-wire with no extra h/w needed (can
actually lose the I2C pullups)

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