RS485 is a very reliable method; I used a casino bus at 57,600bps over a 
long distance (150 m +).
Use the addressable feature of the PIC UART, so that everybody on the 
bus only has to listen to
the address rather than the message body as well.

--Bob A

V G wrote:
> Hi all,
>
> I'm currently working on a project for the university which involves
> interfacing several PIC microcontrollers (dsPIC33FJ128GP802) to a PC
> via MODBUS RTU over RS485. Since the microcontrollers will be
> controlling temperature and humidity, they will be subject to
> temperature and humidity fluctuations. I would imagine that a quartz
> crystal would be advantageous in this case due to it's stability.
>
> I am thinking of running the RS485 line at 9600 bits/s. I am
> interested in maintaining a *stable* bus, it doesn't need to be
> particularity fast. Reliability is the highest priority.
>
> I used a simple program that calculates PIC UART configuration
> register values based on the clock frequency and desired bit rate. I
> selected various common frequencies such as 16 MHz, 20 MHz, and so on,
> but all of them resulted in the program calculating a non-zero error
> percentage value (0.16% at 20 MHz and 9600 bits/s). A screenshot of
> the program can be viewed here:
> http://img249.imageshack.us/img249/8761/screenshotno.png
>
> Is this non-zero error value acceptable? Will it cause reliability
> issues? Does it need to be avoided, and if so, how?
>
> The second question is, does anyone know of a good MODBUS stack for
> the PIC? I will most likely be using the C30 compiler for the
> dsPIC33FJ128GP802. I would also like to use UART RX interrupts to
> handle the protocol, but DMA is a possibility too.
>   

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