PIC18F252 RS232 problem

[jseidmann]

I am currently using the CCS compiler (version 3.190). I have several circuit boards each identical in terms of hardware and software and they each have a different address. They are each supposed to listen to the 485 bus and not respond until their are enabled, and then they are the only ones supposed to respond.

Now, the problem occurs infrequently (like 1 in 100 times) that sometimes two boards talk at the same time although one of them is not even enabled! I dont know if there was some sort of bug in the compiler for the 232 communications that I was not aware of. Currently, the configuration for the RS232 is:

#USE RS232(Baud=19200, XMIT=PIN_C6, RCV=PIN_C7, BITS=8, PARITY=N, ENABLE=PIN_RS485, ERRORS)

the PIN_RS485 is declared elsewhere in the program to the corresponding pin which is used to enable the RS485/RS232 communication.

These all seem to work a large majority of the time, but I occasionally have two or so boards talking at the same time which makes my network go bonkers. I've looked through the code and it looks OK, and the compiler handles the enable for the RS485 and what really throws me for a loop is that it doesnt work only a small percentage of the time, not ALL the time even though nothing changes from one event to the next.

Now, the computer that talks to these boards is just a plain VB program that enables one board and asks it for data, gets the response, and then enables another board, so on and so forth.

My main question was if anyone knows some bug that might cause this to happen or may have some other suggestions. you help would be extremely appreciated


--Jon

[asmallri]

Can't answer your question directly as there are too many unknowns. However I have multiple PICs reliably sharing a RS485 bus but have taken a different approach to addressing the devices. I can address each device in-and (not using RS485 address) and the designated device will respond immediately. This has worked reliably without any overlap. I also use a single broadcast command to interogate all slaves simultaneously. Each slave starts a "window" timer unique to the serial number (address) of the slave. When a slaves timer expires it gains access to the bus.
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Regards, Andrew

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[jseidmann]

Do you enable them separately or always together and they respond in that window? How do you handle it?

[asmallri]