Modbus is a serial communications protocol firstly published by Modicon (at present Schneider
Electric) in 1979 for use with its programmable logic controllers (PLCs).
Simple and robust, it has since become a de facto standard communication protocol, and
it is now a normally available means of connecting industrial electronic devices.
In this video we will learn, What is Modbus used and How does it work?
their advantages and disadvantages.
What is Modbus used for?
Modbus is an open protocol, meaning that it's free for manufacturers to build into their
equipment without having to pay royalties.
It has become a standard communications protocol in engineering industry, and is now the most
normally available means of connecting industrial electronic devices.
It is used widely by many manufacturers throughout many industries.
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 motor speed
and torque and communicates the results to a computer.
Modbus is often used to connect a administrative computer with a remote terminal unit (RTU)
in administrative control and data acquisition (SCADA) systems.
Versions of the Modbus protocol exist for serial lines (Modbus RTU and Modbus ASCII)
and for Ethernet (Modbus TCP).
How does Modbus work?
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 an ones as negative.
The bits are sent very quickly.
A typical transmission speed is 9600 baud (bits per second).
Advantages of modbus.
It’s openly published and royalty-free.
Modbus is developed with industrial applications in mind.
It’s easy to deploy and maintain.
It’s moves raw bits or words without placing many restrictions on vendors.
Disadvantages of modbus.
· Modbus was designed in the late 1970s to communicate to programmable logic controllers,
the number of data types is limited to those understood by PLCs at the time.
Large binary objects are not supported.
· Modbus is restricted to addressing 247 devices on one data link, which limits the
number of field devices that may be connected to a master station (once again Ethernet TCP/IP
proving the exception).
· No standard way exists for a node to find the description of a data object, for example,
to determine if a register value represents a temperature between 30 and 175 degrees.
· Since Modbus is a master slave protocol, there is no way for a field device to “report
by exception” (except over Ethernet TCP/IP, called open-mbus).
· Modbus transmissions must be contiguous which limits the types of remote communications
devices to those that can buffer data to avoid gaps in the transmission.
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