Hi,

Take a look on www.modbus.org; guess you'll find sufficient info on the protocol structure there to write some code.

If you want to test you MODBUS slave I can recommend to get ModPoll from the internet (you'll find it at www.wittecom.com). You can use it for free for a limited period and limited time, but for only $50 you can register. I use it and for the price it's hard to make it yourself. Besides, it's better to test you own code against someone elses to prevent 'dual errors' from believing your code is error-free.

Have fun.