Modbus is a serial master/slave protocol. Modbus can run over any serial type media (copper, fiber, wireless, modem, etc..) Modbus Plus is a peer-to-peer protocol which runs at 1 mbs. The Modbus Plus protocol specifies the software layer as well as the hardware layer. Proprietary cabling and terminators must be used with a Modbus Plus network.
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Modbus Messaging protocol is an Application layer (OSI layer 7) protocol that provides client/server communication between devices connected to different types of buses or networks. The Modbus Messaging protocol is only a protocol and does not imply any specific hardware implementation. Also note that the Modbus Messaging protocol used with Modbus Serial is the same one used with Modbus Plus and Modbus TCP.
Modbus messaging is based on a client/server model and employs the following messages:
- Modbus requests, i.e. the messages sent on the network by the clients to initiate transactions. These serve as indications of the requested services on the server side
- Modbus responses, i.e. the response messages sent by the servers. These serve as confirmations on the client side
The interaction between client and server (controller and target device) can be depicted as follows. The parameters exchanged by the client and server consist of the Function Code (‘what to do’), the Data Request (‘with which input or output’) and the Data response (‘result’).
Modbus (or to be more exact; the Modbus Messaging protocol) is just a protocol, Modbus Plus is a complete system with a predefined medium and Physical layer (OSI layer 1) implementation. It is a LAN system for industrial control applications, allowing networked devices to exchange messages for the control and monitoring of processes at remote locations in the industrial plant. Modbus Plus uses a token-passing medium access control mechanism, which results in deterministic operation, albeit not necessarily fast under all conditions
The Modbus Plus layer 7 messaging protocol is essentially the same as that used for Modbus Serial and Modbus/TCP. The Physical layer is implemented with RS485 and functions over shielded twisted pair cable. The Data Link layer (layer 2) protocol is based on the ISO/IEC 3309:1991 HDLC (High-level Data Link Control) multi-drop protocol, which uses a token passing medium access control mechanism and transmits data in a synchronous fashion as opposed to the asynchronous transmission of Modbus Serial. This results in transmission of data at 1 Mbps
Unlike Modbus, Modbus Plus is a proprietary standard developed to overcome the ‘single-master’ limitation prevalent in Modbus Serial.