What is modbus protocol ?

Modbus is communication protocol first brought up in industry by Modicon. It is use to transfer information between two devices out of which one is master and other is slave. Master ask for information to slave in set messaging format which includes device id, type of function, register start address and length and its cycle redundancy check code and slaves responds to masters request.

It can be based over serial communication like RS 485 or ove internet. When its over serial it is Modbus RTU and when its over internet it is Modbus TCP\\IP. However Modbus is real time protocol.  

MODBUS Protocol is a messaging structure, widely used to establish master-slave communication between intelligent devices. A MODBUS message sent from a master to a slave contains the address of the slave, the 'command' (e.g. 'read register' or 'write register'), the data, and a check sum (LRC or CRC). Since Modbus protocol is just a messaging structure, it is independent of the underlying physical layer. It is traditionally implemented using RS232, RS422, or RS485

Modicon programmable controllers can communicate with each other and with other devices

over a variety of networks. Supported networks include the Modicon Modbus and Modbus Plus

industrial networks, and standard networks such as MAP and Ethernet. Networks are accessed

by built-in ports in the controllers or by network adapters, option modules, and gateways that

are available from Modicon. For original equipment manufacturers, Modicon ModConnect

partner programs are available for closely integrating networks like Modbus Plus into

proprietary product designs.