Modbus is a communication protocol intended for work with programmed logic controllers. Its founder is Modicon Company which at present belongs of French Schneider Electric company. Given specification for the first time became accessible in 1979. The most of developers of electronic components accepted its and started to issue devices for industrial automation supporting this standard. Widespread acceptance of the protocol is caused by its simplicity: many practicing specialists possess knowledges Modbus. Today for development of the specification Modbus-IDA association answers.
Modbus standard relates to application layer of network model OSI. Is based on client-server architecture. Handles the transactions and works on the basis of request/response. For data exchange uses communications links RS-232, -422, -485. As a general rule, in the network one customer and a few servers is planned. The first has «master» status, the others — «slave». The customer transmits enquiries, and the servers answer or execute determined action. The main device can address to all subordinate at once or to one particular. One of the main parts of the Modbus protocol — is PDU toolkit. It contains the feature code and datafield. Its maximum size — is 253 bytes.
Modbus protocol is realized in three variants:
• TCP — for interchange data in TCP/IP networks;
• ASCII — is a textual byte of the message which appears as two hexadecimal characters;
• RTU - numeric, information is transmitted in binary form.
The last specification has clear advantage on the data rate and efficiency.
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