This section explains Modbus.


Modbus is a network developed by an American company, Modicon Inc. for use with PLCs. Its specification is publicly available, so this network has been adopted for general use in the FA industry. Unlike other network standards, Modbus is not managed by any organization. Therefore, it is necessary to check whether compatible devices can be connected to each other using actual devices.

Wiring Method and Communication Protocol

Wiring method

Modbus specifies only the communication protocol and not the communication cables. When only Modbus is indicated, devices are commonly connected according to a serial communication standard such as RS-232C and RS-485. Modbus TCP may be specified when communication is performed using Ethernet.

Communication protocol

Using the master/slave method for communication, only the master can start communication. Multiple slaves can be connected and a command can be sent to them simultaneously. When slaves receive a signal, each responds only when the command is addressed to it and does not respond in other cases. The transmission format consists of a station number, data, and check code.
Transmission data is an ASCII character string or binary data, which can be selected by selecting the transmission mode. American Standard Code for Information Interchange (ASCII) mode is selected to use ASCII character strings and remote terminal unit (RTU) mode is selected to use binary data. The same transmission mode needs to be used among connected slave stations.


Modbus communicates using the master/slave method, in which communication is performed only when the master sends a command.
In typical Modbus communication, reading and writing commands are performed in four types of data areas: the coil, input status, input resistor, and holding resistor. Function codes used for reading and writing in each data area are predetermined. In addition to predetermined function codes, user-defined function codes can also be used.

Function code example
Function code Function
01 Coil reading
02 Input reading
03 Holding register reading
04 Input register reading

Related Networks

Modbus TCP

This is an Ethernet (TCP/IP) based network.

Modbus RTU

This is a network that transmits binary data.

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  • Note that some information, such as applicable standards and specifications, may have changed since this page was published.

November 2015

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