How do you communicate to a Modbus device within an iPhone application?
The big question is how you intend to physically connect to the modbus interface. If you are bridging to it via wifi or ethernet then the bridge will almost certainly have a command set specific to that bridge, and you would use that. If you have a serial bridge you want to connect directly to the iPhone you need to sign up for the Made for iPhone and sign some extra NDAs at which point Apple will give you the details necessary to interface with the USB port, then you build an adapter that has a USB-serial bridge and pump serial commands over it.