I've implemented many modbus nodes and use only 8bits no parity 1 stop bit. The PLCs I also use (TWIDO & Schneider) accept this also. You might wish to check the equipment you want to connect to. To receive modbus rtu I reload timer2 with the required timeout (3 char times) each time I receive a char. When timer2 times out, I have it activate a task to process the received packet. This task checks the CRC (use the table driven crc check it's much faster) and then checks for the slave address then the command. For each of the commands (I only support two - read regs and write regs) I then check the register number and number of registers then do what is required. I create a structure of 'modbus registers' and access them through read and write routines. Therefore in my application code to read or write a modbus register I write something like a= read_modbus_reg(MB_LEVEL);
MB_LEVEL being one of my defined modbus registers. I've never got around to polishing my code for publication but I can post the code if you wish.