Open Master / Slave protocol

Modbus was originally developed by Modicon and today, it is managed by the Modbus User Organization. Modbus is an open Master/Slave application protocol, that can be used on several different physical Layers. Modbus is an application-layer messaging protocol, positioned at level 7 of the OSI model. It provides client/server communication between devices connected on different types of buses or networks. Modbus-RTU (Remote Terminal Unit) means that the Modbus protocol is used on top of a serial line with an RS-232, RS-485 or similar physical interface. Numerous automation systems have Modbus-RTU interfaces for communication. Modbus devices are certified by the Modbus User Organization for interoperability and conformance to the Modbus specification.

When devices communicate on a Modbus serial line using the RTU mode, each 8–bit byte in a message contains two 4–bit hexadecimal characters. The main advantage of this mode is that its greater character density allows better data throughput than ASCII mode for the same baud rate. Each message must be transmitted in a continuous stream of characters.

MODBUS is a request/reply protocol and offers services specified by function codes. MODBUS function codes are elements of MODBUS request/reply PDUs. Modbus provides a set of functions to read and write data in the field devices. Modbus supports bit or word data transfers. In RS-485 based Modbus networks, the Address Field in the telegram frame is used to address a single device in the network.

Modbus-RTU FACTS
Network Type:	Simple Master/Slave communication system
Topology:	RS-232: peer-to-peer connection between Master and Slave RS-485: line topology with segments of up to 32 devices. Each                 segment shall be terminated at the Beginning and End.
Installation:	Shielded twisted pair cables. Line length depending on physical media and baudrate:
Data Rate :	User selectable; depending on physical media
max. Stations :	1 Master and up to 246 Slaves
Data:	0 - 252 Byte per telegram frame
Network Features :	Simple Master/Slave network for peer-to-peer (RS-232) or Multidrop (RS-485) communication.
User Organization:	Modbus-IDA user Group