Modbus Protocol Specification
Figure 1 Overview of Modbus Protocol Application
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| Figure 1 Overview of Modbus Protocol Application
1.1.1 Transactions on Modbus Networks Standard Modbus ports on Modicon controllers use an RS-232C compatible serial interface that defines connector pinouts, cabling, signal levels, transmission baud rates, and parity checking. Controllers can be networked directly or via modems. Controllers communicate using a master-slave technique, in which only one device (the master) can initiate transactions (queries). The other devices (the slaves) respond by supplying the requested data to the master, or by taking the action requested in the query. Typical master devices include host processors and programming panels. Typical slaves include programmable Modbus Protocol http://www.modicon.com/techpubs/intr7.html (2 of 5) [1/11/2000 10:36:08 PM] controllers. The master can address individual slaves, or can initiate a broadcast message to all slaves. Slaves return a message (response) to queries that are addressed to them individually. Responses are not returned to broadcast queries from the master. The Modbus protocol establishes the format for the master's query by placing into it the device (or broadcast) address, a function code defining the requested action, any data to be sent, and an error-checking field. The slave's response message is also constructed using Modbus protocol. It contains fields confirming the action taken, any data to be returned, and an error-checking field. If an error occurred in receipt of the message, or if the slave is unable to perform the requested action, the slave will construct an error message and send it as its response. 1.1.2 Transactions on Other Kinds of Networks In addition to their standard Modbus capabilities, some Modicon controller models can communicate over Modbus Plus using built-in ports or network adapters, and over MAP, using network adapters. On these networks, the controllers communicate using a peer-to-peer technique, in which any controller can initiate transactions with the other controllers. Thus a controller may operate either as a slave or as a master in separate transactions. Multiple internal paths are frequently provided to allow concurrent processing of master and slave transactions. At the message level, the Modbus protocol still applies the master-slave principle even though the network communication method is peer-to-peer. If a controller originates a message, it does so as a master device, and expects a response from a slave device. Similarly, when a controller receives a message it constructs a slave response and returns it to the originating controller. tải về 0.61 Mb. Chia sẻ với bạn bè của bạn:
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