Practical Industrial Data Networks:Design, Installation and ...

8Modbus Plus protocol overviewObjectivesWhen you have completed study of this chapter, you will be able to:8.1 General overview• Describe the main features of Modbus Plus• Fix problems with:• Cabling• Grounding/earthing• Shielding• TerminationsBesides the standard Modbus protocol, there exist two other Modbus protocol structures:• Modbus Plus• Modbus IIThe more popular one is the Modbus Plus. It is not an open standard as the classicalModbus has become. Modbus II is not used much due to additional cabling requirementsand other difficulties.Modbus Plus protocol was developed to overcome the ‘single-master’ limitationprevalent in the Modbus Protocol. As described earlier, in order to share informationacross various Modbus networks, a designer would have had to employ a complexhierarchical network structure in order to achieve system wide networking.Modbus Plus is a local area network system for industrial control applications.Networked devices can exchange messages for the control and monitoring of processes atremote locations in the industrial plant. The Modbus Plus network was one of the firsttoken-passing protocol networks that pioneered the development of other more advanceddeterministic protocols today.The Modbus Plus protocol topography allows for up to 64 devices on a networksegment. Each device on a network segment must be assigned a unique network addresswithin the range 01 through 64 inclusive. Multiple networks segments may be bridgedtogether to form large systems. Messages are passed from one device to another by the