John, I'm glad you were able to use hints from my code. Sometimes looking at other people's code confuses the issue - you are normally better off to start with the specification, then look at other people's code to see how they solved the problem. Would you agree that after the fact that it is quite simple to implement Modbus?

I'm looking at doing some USB stuff at the moment - in terms of a protocol - it is much more complex. Trying to solve it from looking at the datasheets for the host controller etc only confuses you more as it uses specific terms. Understanding the operation from the specification introduces these terms and explains them, so the datasheet makes a whole lot more sense.