Stewart,

THE Modbus website is www.modbus.org. There you'll be able to find all the specs you need to create a MODBUS master or slave.

Having said that, and taken the previous posts into consideration, I might be able to add some comments that may be of use.

MODBUS originates from way back in the eighties when PLC's were still only used as relay-replacing controllers and only touched the area of data acquisituon and -processing. In those good-old Dino days, a PLC would process bits and bytes, not FP's or strings.

That's what MODBUS was based upon, but unlike the Dino's it just doesn't seem to be willing to die. So it has been adapted to suit the needs of modern men, but has the appearance of a schoolbus in bikini ...

The basic (and original) MODBUS protocol supports transfer of bits, bytes and (16-bit) words, either single or in multiple (refer to the protocol). The contents of each bit, byte or word is basically irrelevant to the protocol itself. A single byte could be just an unsigned number, or an ASCII character, but may just as well be the least significant byte of a 32-bit IEEE floating point: it is what you want it to be.

My suggestion is: get hold of the data sheet, read it and post any questions that you may have. Most important thing is that you understand the basics. When it comes to the coding: that will be the least problem.

Menno.