Does anyone know of a software package that can be used to scan through all possible Modbus addresses and retreive the address of the device connected?
All of Modicon's programming software packages have the ability to scan communications networks. When you try to connect, go on-line, to a PLC you have the choice of scanning for conected devices. You can download demo versions at http://www.modicon.com. I know this is probably more than you want but it is one idea.
Try this website. I have used the Modscan32 for exactly what you are trying to do. This link is for free demos, but the licensed version is also worth it. Modscan32