How to build a Modbus driver in Go?

Oh wow you actually implemented transaction and protocols IDs? Most libraries don't bother. I know the few I use in production don't. They /should/ be used for language extensions, but generally they aren't.

Also not to be a bit pedantic but MODbus/TCP is a super set of MODbus/RTU. The distraction between ASCII and RTU is large, but existent because the size of a "byte" is variable in serial communication. I.E.: You can't use MODbus/RTU with a 7 DataBit serial connection.

Also if you get bored point your library at a good data center cisco router, the results are fun ;)