I'd like to have a serial (one way would be okay, two way better) interface
between two PICs with different power supplies.  One would probably be
on a regulator powered by a wall wart and the other would be battery
powered.  The battery powered device will be on a rotating platform,
so I'd like to connect just through the metal axle it's rotating on.

I'm a software guy, not an EE guy, so if this idea is stupid for some
obvious reason, please enlighten me.  Here's my circuit (one half, one
for each side):

                 VCC
                  +
    TX    R1_     |
    o----|___|----|-----.
          100     |     |
                  |     |  C1
                   \|   |  ||  WIRE
                Q1  |---o--||---o
                   <|      ||
    RX            |       1uF
    o-------------o
                  |
                 .-.
              1k | |R2
                 | |
                 '-'
                  |
                 ===
                 GND

Q1 acts as an envelope detector and provides a logic level signal to RX.
TX is high-Z when receiving.  When transmitting, RX sees what got sent
and the 'slave' device can release the line if it sees an error.

TX needs to be driven without a DC component, which can probably be done
by manchester encoding bytes before sending them via the UART (start/stop
are already balanced).  It might be necessary after a quiescent period
to drive 0xAA (or 0x55) to frame the message and 'balance' the line.

--
Ben Jackson
<ben@ben.com>
http://www.ben.com/

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