Modbus is a versatile and proven communication protocol that is widely used in industrial automation as well as other applications, as it enables easy and efficient data exchange between devices on a network. The two main variants, Modbus TCP and Modbus RTU, differ in transmission technology and frame structure, so they can meet different application areas.
The communication interface “Modbus Transmission Control Protocol” and is a variant based on the Internet Protocol (IP). It uses the standard TCP/IP network found in most modern industrial and commercial networks. In addition, data can be transmitted via Ethernet or wireless Wi-Fi connections, which ensures fast and reliable communication. In the interface protocol, data is transferred in the server-client model in the form of Ethernet frames. These frames contain the commands and data of the interface that are exchanged between a client (e.g. a PC) requesting the data and a server (e.g. a controller). Data is transmitted via port 502, which allows for easy integration into existing network infrastructures.
The advantage of Modbus TCP lies in its simplicity and interoperability. Since it is based on TCP/IP, it can easily communicate over LANs, WANs, or the Internet. It is also well suited for applications where fast and reliable data transmission is required, such as industrial automation and building automation.
Another communication interface, the “Modbus Remote Terminal Unit”, is a serial variant of the protocol. Unlike Modbus TCP, which uses Ethernet, Modbus RTU uses a serial interface such as RS-232 or RS-485 for data communication. In this case, the transmission of data takes place in the form of binary values. This makes them more efficient, as there is no need for extensive header information, as is the case with Modbus TCP. Communication occurs in a master-slave model, with the master sending the commands and the slaves receiving the data or responding to the commands with data.
One of the main advantages of this communication interface is the robustness of serial communication, which is consequently advantageous in harsh industrial environments. The short data transfer time and easy cabling make it a practical choice for applications where a longer distance between devices is required, such as in large factories or buildings.
The third communication interface presented in this article is called “Modbus RTU encapsulated in TCP” and is another variant of the Modbus protocol. Their main purpose is to transport serial RTU traffic over a TCP/IP network. The serial data is encapsulated in TCP/IP packets and then sent over the Ethernet. This approach makes it possible to take advantage of both protocol variants. To use this interface, a so-called “Modbus Gateway” is usually used. A gateway is a device or software that is positioned between the Modbus TCP master and the serial RTU slaves. It takes on the task of converting the RTU messages into TCP/IP packets and vice versa. This allows RTU devices to be seamlessly integrated into a TCP network without the need for a complete transition to Modbus TCP.