(Sorry, newbee error, resent with correct TAG [PIC])

I have a need to design a circuit that controls four RS232 serial ports and
a parallel printer port. The four serial ports would be connected to a 3COM
Palm Pilot (Main control), a single board modem, an external serial printer
and an external PC serial port. Any serial port can talk to any other serial
port. e.g. Palm to PC, Palm to Modem, PC to Modem etc ....

In order to do this I propose using a 16F877 as the master PIC controlling a
(Palm) serial port and the parallel printer port. To control the other three
RS232 serial ports I propose using three slave PIC's 16F627 because they are
cheap (2.22UKP). The Master PIC would then be in communications with the
three slave PIC's.

Question:
1. Has anyone got any advice on PIC to PIC communications using just two
lines (RX, TX).
2. Has anyone done it before? (Bound to have)
3. Anyone got a better idea (see Design Considerations below)
4. Rather than a Master PIC would a PIC token ring be better?
5. Anyone played with 16F627's, any problems with them as far as RS232 is
concerned?

Design considerations:
1. I only want to download the Master PIC inorder to commission the circuit
(using incircuit methods). Therefore the Master PIC must itself incircuit
download the three Slave PIC's. I would imagine the three slave PIC's
containing identical programs. The circuit must self test itself every hour
and take corrective action if possible. This may require the PIC to takeover
the modem and dial for help!
2. I can spare 9 I/O lines in the master PIC for connection to the three
slave PIC's. Therefore each slave PIC would have 3 connections, its  RB6,
RB7 and RA5 ( wired to the Master PIC. This would allow incircuit download
of the slave PIC's.
3. The Slave PIC's would then have two lines RB6 and RB7 for receiving and
transmitting data to and from the Master PIC.
4. I could clock all four PIC's from the same source to make the
syncronisation easier.
5. The main control program will run in the Master PIC. The Slave PIC's
interrupting the Master PIC.
6. The Slave PIC's RAM memory would be used as a chash, buffering 128 bytes
inbound and 16 bytes outbound.
7. The Slave PIC's may be communicating at 115K baud, but would typically be
communicating at 38K baud.
8. The Master PIC is acting as a serial to parallel converter. (As well as
the main switch interface between the PIC's)
9. The parallel printer port is actually two parallel ports. An incomming
one from a PC and an outgoing one to a printer. This allows the Palm to
disconnect the PC parallel port and take over (borrow) the PC's parallel
printer.

regards
David Clavey

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