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Modbus is a serial communications protocol originally published by Modicon (now Schneider Electric) in 1979, for use with its programmable logic controllers (PLCs). Simple and robust, it has since become a defacto-standard communication protocol, and it is now a commonly available means of connecting industrial electronic devices. The main reasons for the use of Modbus in the industrial environment are:
Modbus enables communication with many (approximately 240) devices connected to the same network: for example a system that measures temperature and humidity and communicates the results to a computer. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Many of the data types are named from their use in driving relays: a single-bit physical output is called a coil, and a single-bit physical input is called a discrete input or a contact.
The development and update of Modbus protocols has been managed by the Modbus Organization since April 2004, when Schneider Electric transferred rights to that organization. The Modbus Organization is an association of users and suppliers of Modbus-compliant devices that advocates for the continued use of the technology.
Understanding Modbus Serial & TCP/IP
What is Modbus?
Modbus is a serial communication protocol developed by Modicon published by Modicon® in 1979 for use with its programmable logic controllers (PLCs). In simple terms, it is a method used for transmitting information over serial lines between electronic devices. The device requesting the information is called the Modbus Master and the devices supplying information are Modbus Slaves. 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.
What is it 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 industry, and is now the most commonly 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 temperature and humidity and communicates the results to a computer. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory 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 is data stored in Standard Modbus?
Information is stored in the Slave device in four different tables. Two tables store on/off discrete values (coils) and two store numerical values (registers). The coils and registers each have a read-only table and read-write table.
Each table has 9999 values.
Each coil or contact is 1 bit and assigned a data address between 0000 and 270E.
Each register is 1 word = 16 bits = 2 bytes and also has data address between 0000 and 270E.
What is a Modbus Map?
A modbus map is simply a list for a slave device that defines - what the data is (eg. pressure or temperature readings) - where the data is stored (which tables and data addresses) - how the data is stored (data types, byte and word ordering)
Some devices are built with a fixed map that is defined by the manufacturer. While other devices allow the operator to configure or program a custom map to fit their needs.