The unlike Modbus Serial and Modbus TCPIP the Modbus Plus protocol has not been openly published.
If you would like to implement Modbus plus on a computer please refer to the Modbus Plus interface cards from Schneider Electric - they are supplied with drivers etc. OR you may use software like the OPC server.
Modbus-Plus is a 1Mbs, peer-to-peer, token passing network that carries Modbus/RTU messages. The one byte slave address used in
Modbus/RTU is replaced by a five byte routing path.
Windows drivers for the Modbus-Plus adapters are available from Modicon. Windows OPC drivers are available from a number of sources. Drivers for QNX and Unix are available from 3rd parties. If you want to make your own host adapter I think you should talk to Modicon about buying the Modbus-Plus chip set, licensing, and a Modconnect parternship agreement.