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Tuesday, 3 April 2012

Modbus Communication

Modbus is a serial communications protocol published by Modicon and generally used with PLCs(programmable logic controllers). It has now become a factory standard for communications protocols in industry, and is the most commonly used means of communicating industrial devices.

The main reasons for the extensive use of Modbus over other communications protocols are:

  •  It is open source protocol and it’s free.
  •  It is simple to understand, easy to implement.
  •  It moves raw bits or words without placing many restrictions on vendors

 Modbus allows for serial communication between many third party devices connected to the same network, for e.g. a system that measures temperature and humidity and communicates the results to a computer or number of VFDs communicating to PLC over a single Modbus serial line. 

Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Modbus protocol is available for serial and Ethernet ports.

Each device intended to communicate using Modbus is given a unique address. Maximum of 247 stations can be connected on a Modbus network. Maximum length of wire is limited to 1200mtr. Generally Master/slave configuration used for Modbus. 

Any device can send out a Modbus command, although usually only one master device does so. A Modbus command contains the Modbus address of the device it is intended for. Valid range for address is 0 to 247. An address equals to 0 means a broadcast to all devices in the network. Only the intended device will act on the command, even though other devices might receive it. All Modbus commands contain checking information, ensuring that a command arrives undamaged. The basic Modbus commands can instruct an RTU to change a value in one of its registers, as well as commanding the device to send back one or more values contained in its registers.

Governing Standard(s)
EN1434-3 ,IEC870-5

Communication Methods
Communication Speed
9.6 -115.2 kb/s
Max. Data Size
250 bytes (125 register)
Max. Stations
Maximum Cable Length
1200 mtr.
For Reference only
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