If all of the following are true:

- The PC is going to act as a master

- The F877's will only speak when spoken to

- The F877's are physically  close to each other (ideally sharing the same
power supply).

Then there is a low-cost simple way to be your network...

Take the TD signal from the PC and route it thru the RS232 to TTL channel of
the MAX220 to the RD signals of all four PICS.

Connect a signal diode (IN4148 is fine) from the TD pin of each of the PICs
to the TTL to RS232 channel of the MAX220. The cathode of the diode should
be connected to the PIC pin.

Connect a 10k resistor from the junction of all the diodes with the MAX220
pin to the positive supply rail.

Connect the output of the TTL to RS232 channel of the MAX220 to the PC.

All the F877's will be able to 'hear' the PC.

The PC will be able to 'hear' any F877.

As long as only one F877 at a time 'talks' everything will be well.

If more than one F877 tries to talk at the same time the PC will get garbled
data, but no hardware damage will be done.


Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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