IMHO you will have to reinvent the wheel. Everybody talking about it, but nobody to explain it. I am looking for basic explanation too, but without success. Is it some kind of secret!? Everybody puts the link The Modbus Organization
and say that is easy to understand. To me, it is not.
I wish you luck to find explanation, or try to write your own protocol, its easiest way and you will know exactly what you done :-)