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What is Modbus? Explain its working.

Subject : Embedded Systems Design

Topic : Embedded Serial Communication

Difficulty level : Medium

1 Answer
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i) Modbus is a serial communication protocol developed by Modicon published by Modicon® in 1979 for use with its programmable logic controllers (PLCs).

ii) In simple terms, it is a method used for transmitting information over serial lines between electronic devices.

iii) The device requesting the information is called the Modbus Master and the devices supplying information are Modbus Slaves.

iv) In a standard Modbus network, there is one Master and up to 247 Slaves, each with a unique Slave Address from 1 to 247. The Master can also write information to the Slaves.

v) Working of Modbus –

  • Modbus is transmitted over serial lines between devices.
  • The simplest setup would be a single serial cable connecting the serial ports on two devices, a Master and a Slave.
  • The data is sent as series of ones and zeroes called bits.
  • Each bit is sent as a voltage. Zeroes are sent as positive voltages and ones as negative.
  • The bits are sent very quickly. A typical transmission speed is 9600 baud (bits per second).

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vi) Modbus is an open protocol, meaning that it's free for manufacturers to build into their equipment without having to pay royalties.

vii) It has become a standard communications protocol in industry, and is now the most commonly available means of connecting industrial electronic devices. It is used widely by many manufacturers throughout many industries.

viii) Modbus is typically used to transmit signals from instrumentation and control devices back to a main controller or data gathering system, for example a system that measures temperature and humidity and communicates the results to a computer.

ix) Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems.

x) Versions of the Modbus protocol exist for serial lines (Modbus RTU and Modbus ASCII) and for Ethernet (Modbus TCP).

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