If you want to write a ModBus master program for the PC, using .NET, this may be for you.
Circuit Cellar magazine have run a 2-article series by Aubrey Kagan, which goes right through the process of designing and writing a generic ModBus Master system. If you don't subscribe to Circuit Cellar, you can buy the two articles for $1.50 each at the Circuit Cellar website. They appeared in issues 200 (March 2007) and 201 (April 2007). I've tried to extract simple links to the articles for you, but the website is not structured to allow that (it is otherwise a really good, comprehensive website, as befits an excellent technical journal). So you will have to dig a bit.
Disclaimer: I've not tested this program, so I can't vouch for it. Circuit Cellar has a reputation for good stuff.
If you are using the older VB6, you can look at my ModBus program, which installs with SPLat/PC (dig in the examples folder).
