Computer Busses

18

Modbus

18.1 Modbus protocol

The Modbus protocol is an industry-standard protocol which allows programmable controllers to communicate over a network or local communications link. It defines a standard message structure that all Modbus-compatible controllers recognise and implement, regardless of the network type. It describes:

• The format of requests to Modbus-compatible devices.

• The format of responses from Modbus-compatible devices.

• The layout and contents of message fields for Modbus-compatible devices. The Modbus protocol provides the internal standard that the Modicon controllers use for parsing messages.

• How each controller knows its own device address and recognizes any messages addressed to it.

• The format of the data and other information ...