A.3 Key feature

There are many devices and gateways that support Modbus, as it is a very simple protocol and convenient to transmit and understand. Specially, Modbus TCP/IP simply takes the Modbus instruction set and wraps TCP/IP around it. Development costs are exceptionally low. Minimum hardware is required, and development is easy under any operating system. The following are key features of Modbus:

• Communication mode
  Modbus uses master-slave/client-server communication mode, Master issues a unicast request and slave responds to that. In serial and MB+ networks, only the node assigned as the Master may initiate a command. On Ethernet, any device can send out a Modbus command, although usually only one master device does so. Modbus also supports broadcast mode where master's request is sent to all the slaves but no slave responds to broadcast request.
• DData model
  Modbus manages the access of data simply and flexibly. Modbus data are divided into four ranges, they are that these types of data can be provided/alterable by I/O system or an application program. In most cases, slaves store each type of data that it supports in separate memory, and limits the number of data elements that a master can access.
• FFunction code There are three categories of Modbus Function codes, including Public Function codes, User-Defined Function codes and Reserved Function codes. Public Function codes can satisfy common operations, such as accessing data in device by reading and writing data model, and simply diagnosing device. Function code is flexibility that user can select and implement a function code by self-defining User-Defined Function codes according to service requirements.
• Availability of many devices Interoperability among different vendors' devices and compatibility with a large installed base of Modbus-compatible devices makes Modbus an excellent choice.