Hi,
Beg Pardon for english - not my native language.

I used max232 + sn75176 for the relationship COM-a port with 14
controllers
on pic84 on speed 9600 and 19200 bps on the distance 250 metres.
Protocol of exchange has stolen from the cash device:
1 word - "AAh" - start
2 - amount a byte in frame (N)
3 - command
4 - address of device
5 - sent data ( if command - record )
......
N - inv control total of all bytes.

Callable controller will send a byte "OK" - 5Ah or not - "OK" - A5h or
nothing. Hereinafter - executes a command. Back, issue goes in same
type.
If received not - "OK" or no answer, repetition of issue else 3 times.
Controllers work at the street, not switching off more.

Willingly divide lifelengths on pic84

VADIM vad@anet.lv