Thanks Russell

I to was doing it this way although I only support Modbus RTU and I have a similiar state machine principle. Where by I perform what ever action I am commanded to do.

I then compile a reply message in a typdef struct, run a CRC on the entire structure and send it off.

The problem comes when a device reads a whole heap of registers (in my case I am running 30 odd 16bit counters) while I happily store these in RAM (using 60bytes) I don't really want to copy them across to my struct to then send it out the serial port. that uses another 60bytes (the order is all different) also the customer for whatever reason doesn't want them in consequtive modbus registers, so I was going to fill the blanks in on the fly.

In short the way that I am doing it now - as per my previous posts - is to use a ring buffer on the serial port and fill the port buffer a little at a time keeping a runningCRC for each byte sent.

So i do this (psuedo code)
SendByte(Address)
runningCRC(Address, 1bytetocalc, 0xFFFF CRC init)

Sendbyte(FunctionCode)
runningCRC(FunctionCode, 1bytetocalc, lastCRC)

SendByte(DataBytes)
runningCRC(DataBytes, 1bytetocalc, lastCRC)

SendByte(DataHi)
runningCRC(DataHi, 1bytetocalc, lastCRC)

SendByte(DataLo)
runningCRC(DataLo, 1bytetocalc, lastCRC)

:
: ~60bytes in here
:

SendByte(DataHi)
runningCRC(DataHi, 1bytetocalc, lastCRC)

SendByte(DataLo)
runningCRC(DataLo, 1bytetocalc, lastCRC)

SendByte(RunningCRC Lo Byte)

SendByte(RunningCRC Hi Byte)

This is not quite a s pretty as lading the structure and then sending it all off in one bang but it does mean that I save on 60bytes of RAM.

Any further ideas appreciated.

Regards

Marshall