industrial//industrial communication//fieldbus//Modbus//Modbus TCP

Modbus TCP is the variant of the Modbus protocol that runs over TCP/IP, normally on Ethernet, so that Modbus devices can be reached across an ordinary network. The registers and function codes are those of Modbus RTU; what changes is the transport: devices are identified by IP address instead of a number on a shared serial bus and can sit behind ordinary switches with many other hosts. Three identifiers meet in one frame and are easy to confuse: the MAC address names the network interface on the local link, the IP address names the host across networks, and the Modbus unit identifier names a device behind a gateway that serves several serial devices through one IP. A gateway that maps one onto the other therefore changes nothing in the meaning of the data.

Modbus TCP is the variant of the Modbus protocol that runs over TCP/IP, normally on Ethernet, so that Modbus devices can be reached across an ordinary network. The registers and function codes are those of Modbus RTU; what changes is the transport: devices are identified by IP address instead of a number on a shared serial bus and can sit behind ordinary switches with many other hosts. Three identifiers meet in one frame and are easy to confuse: the MAC address names the network interface on the local link, the IP address names the host across networks, and the Modbus unit identifier names a device behind a gateway that serves several serial devices through one IP. A gateway that maps one onto the other therefore changes nothing in the meaning of the data.

It is the simplest member of the industrial Ethernet family, and its advantage is exactly that: simplicity and interoperability. Almost everything can speak it. It offers none of the sophistication of the control-grade networks: no object model, no scheduled real-time traffic, no device diagnostics. For a drive or a remote I/O rack under a PLC, PROFINET or EtherNet/IP are the usual choice; for reading a meter, Modbus TCP is enough (industrial Ethernet).