Modbus-TCP Industrial Ethernet Protocol Overview

Ethernet TCP/IP Protocol

Modbus was originally developed by Modicon and today, it is managed by the Modbus-IDA 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-TCP means that the Modbus protocol is used on top of Ethernet-TCP/IP. Modbus-TCP is an open Industrial Ethernet network which has been specified by the Modbus-IDA User Organization in co-operation with the Internet Engeneering Task Force (IETF) as an RFC Internet standard. Modbus devices are certified by the Modbus-IDA User Organization for interoperability and conformance to the Modbus specification.

Disparaged by critics but highly valued by those who have used it, Modbus-TCP isn't really anything new. Rather more, it was solely necessary to approve Ethernet-TCP/IP as an additional data transmission technology for the Modbus Protocol, which has been available since 1979. The well-proven Modbus services and the object model which has been available since the original Modbus protocol version are unchanged, and have simply been adapted to TCP/IP as the data transmission protocol. This extends the Modbus family with an additional product range, which now consists of the classical Modbus-RTU (asynchronous data transmission via RS-232 or RS-485), Modbus-Plus (high speed communication via a Token Passing Network) and Modbus-TCP (Ethernet-TCP/IP-based client/server communication). All of these versions share the same application protocol, which specifies a universal object module for user data and communication services.

Modbus is a request/reply protocol and offers services specified by function codes. Modbus function codes are elements of Modbus request/reply PDU's. Modbus provides a set of functions to read and write data in the field devices. Modbus supports bit or word data transfers.

The performance of a Modbus-TCP network is highly dependent on the type and design of the Ethernet network which is used and on the performance of the processors in the communication interfaces of the respective devices.

Modbus-TCP is a pragmatic approach to use Ethernet as a data transmission medium for automation applications. The additional costs of the network infrastructure (star topology with intelligent switches) can be justified by the advantages of Ethernet such as the large number of stations in a network and by substantial benefits due to additional IT functions embedded Internet, email and file transfer) which can use the same medium.

MODBUS-TCP FACTS
Network Type:	Ethernet-TCP/IP based simple Client/Server network
Topology:	Very flexible with star, tree or line structures. All topologies that can be implemented with standard Ethernet technology including switched networks are applicable.
Installation:	Standard 10, 100 or 1000 Mbit/s Ethernet technology based on copper cables, fibre optics or wireless standards can be used.
Speed :	10, 100, 1000 Mbit/s/s
max. Stations :	nearly unlimited
Data:	Up to 1.500 Byte per Telegram frame Total: nearly unlimited
Network Features :	Simple Client/Server network based on standard Ethernet technology and TCP/UDP/IP protocols in Layer 3-4.
User Organization:	Modbus-IDA user Group

HMS is an active member of the Modbus-IDA User Group and provides many certified Modbus-TCP products.