What are the differences between MODBUS PLUS and MODBUS?
What are the preferences of MODBUS or MODBUS PLUS than the others? (compare specifications)
Thank you very much

 

Modbus is a widely used serial protocol. Usually used in a point to point communication (rs232) or point to multipoint (rs485 or rs422).

Modbus Plus is a proprietary multipoint to multipoint 1 mb/sec rs485 network. We installed a modbus plus network over fiber in our plant. This allows our FIX32 to communicate with about 15 modicon PLC's (some 984's some Quantum's). It also allows any PLC to read or write to any other PLC on the network. We might have went to modbus/tcp but we had some older 984-685E's that don't have much support for ethernet. The Modbus Plus works pretty well and we continue to expand on if.

 

Here are a few main differences. Modbus Plus is 1Mbaud and Modbus is maximum of 19.2Kbaud (for all practical applications) Modbus is a open/free protocol that uses standard serial connections such as rs232 or rs485 typically.

Modbus Plus is a proprietary network owned and licensed by Schneider Electric (Modicon) that uses a Token Ring modified RS485 2wire network design. Modbus Plus has Modbus protocol encapsulated within it message makeing it easy for a bridge device to allow the two networks to communicate with each other. For more detail, go to "www.modbus.org":http://www.modbus.org where you can get the complete modbus specification and some limited info on Modbus Plus. You will not find complete Modbus Plus information posted publicly, as you need to be a schneider alliance partner and sign agreements with them before you are allowed to implement your own hardware using this protocol.

You did not ask about Modbus TCP/IP with is an additional open/free network protocol that is essentially the modbus protocol encapsulated in a TCP/IP ethernet block. These protocols are very popular because it is easy and innexpensive to implement products that use the protocols, and easy to convert between the diffent protocols.

 

Modbus is generally used for either geographically distributed systems (they need telephone, radio or microwave connections) or to connect a limited (3-5) node system with limited data requirements. (at 4 nodes you could read about 80 data words per second per node). Often scales or specialty instruments are connected point to point this way.

Modbus Plus offers SPEED, at a price of limited distances (a single segment is limited to 1500 ft on RS485, longer distances can be achieved with fiber optics and/or repeaters). The key difference is that MB+ offers real volumes of data in real time. The network can move more than 2000 data words per second, while supporting programming terminals (PCs). If you do any work with HMI to PLC, you want this network. Also if you program in IEC languages, YOU want this network.