Modbus, is based on the RS232C serial communications standard that defines the connector pinouts, cabling, signal levels, transmission baud rates and parity checking. It is considered a master slave protocol and it is the common language used by all Modicon controllers.
Modbus Plus, is a high speed peer to peer network protocol based on token passing communication. Modbus Plus nodes on a the network access the network upon receipt of a token frame. When a node holds the token it can initiate transactions with other devices on the network.
Extensive detailed documentation and information on these protocols is available in hard copy and at web sites " www.modicon.com " and www.modbus.org.