SD01189C/07/EN/04.17
Endress+Hauser
6 © Copyright Dec. 2016
3 Modbus Protocol
3.1 Specification
Modbus is an open, standardized communication protocol that is used in the areas of Manufacturing,
Process- and Building automation.
The Modbus protocol is used for data exchange with intelligent field devices via master/slave
(client/server) connections. Here, the Modbus protocol can be used with different transmission media and
embedded in other communication protocols.
The specifications for the Modbus protocol are freely available on the web site of the Modbus
Organization.
http://www.modbus.org/
3.2 Basic principle
3.2.1 General
Modbus communication (unicast) uses a simple request-response mechanism.
There are three different scenarios:
If the device (server) receives a valid telegram with a valid request and is able to reply to it, it replies with a
response telegram containing the function code of the request.
If the device (server) receives an invalid telegram (e.g. CRC error), it does not reply. The application (client)
must carry out an error diagnosis once a specified time period (time-out) has elapsed.
If the device (server) receives a valid telegram with an invalid request, it replies with a Modbus exception.
The application (client) must carry out an error diagnosis.
To Next Page
Loading...
