What are the Modbus addresses for the registers?
You need to get these from the manufacturer of the Modbus device. There is no equivalent of an XIF file for Modbus. The register map will exist in the form of documentation you need to obtain from the manufacturer. If you find somebody that knows what Modbus is, they should also know where to find the “Modbus register map”. Once you get your hands on the documentation, the next trick is interpreting it. Some high percentage of the time, all of the registers will simply be holding registers. When this is the case, registers may be documented as starting from zero, starting from one, starting from 40001, or starting from 400001. It is common to document registers starting at #1, but this becomes address 0000 over the wire. Technically, “by the book” or per spec, addresses only start at zero. But their “references” are commonly documented as starting from #1. If you don’t get the response you expect at the address you think you should, try that address plus one and that address minus one.
