Modbus Protocol for Industrial Automation Quick facts

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Modbus is a serial communications protocol published by Modicon in 1979 for use with its programmable logic controllers (PLCs).
Modbus is not industry specific and is used across a wide range of industries.
Modbus is used in multiple master-slave applications to monitor and program devices; to communicate between intelligent devices and sensors and instruments; to monitor field devices using PCs and HMIs.
Although the main Modbus message structure is peer-to-peer, Modbus is able to function on both point-to-point and multidrop networks.

MODBUS is considered an application layer messaging protocol.
Modbus protocol defines a message structure, how the PC addresses and requests information from a Modbus device, how the device will respond and how errors are detected and reported.
On the OSI model, MODBUS is positioned at level 7.
Modbus is a data transfer protocol used mainly in the building and industrial automation industry.
Modbus is now an open standard, administered by the Modbus-IDA
Modbus typically utilizes RS-232 and RS-485 to physically connect to PLC’s, I/O, and other hardware
Modbus is designed to move raw data, regardless of form or function. This gives vendors enormous flexibility in development on their end, and greatly minimizes data format restrictions
Modbus is a master-slave technology. This means that only one device on the network can act as master, while the others act as slaves.
Unlike the Standards RS232, RS422 and RS485, Modbus is a protocol that doesn’t insist on voltage levels, connection pins, etc. Modbus devices can use a variety of cables or networks, including RS485 or RS232.
Modbus has two modes: ASCII (American Standard Code for Information Interchange) and RTU (Remote Terminal Unit).
Modbus can be used over TCP/IP (Transmission Control Protocol/Internet Protocol).
Modbus is a free and open standard, it lowers the cost of development and implementation on all ends.
Modbus is limited to 247 connections per master unit/system. The exception is when TCP/IP mode is used because it makes the ratio between master and device virtually unlimited.
Modbus Protocol Organization can be reached via http://www.modbus.org/