Modbus TCP is an industrial communication protocol that enables devices like PLCs, HMIs, and sensors to communicate over standard Ethernet networks. It combines the proven Modbus messaging format with TCP/IP networking, allowing industrial automation systems to leverage modern network infrastructure for reliable data exchange.
Modbus TCP follows a layered protocol stack architecture. At the top is the Modbus Application Protocol, which contains the actual device commands and data. This is encapsulated within TCP for reliable transmission, then IP for routing across networks, and finally Ethernet for the physical connection. The protocol typically uses port 502 for communication, ensuring standardized connectivity across different industrial devices.
The Modbus TCP message structure is carefully designed for network communication. It starts with a standard TCP header for network routing. The Modbus Application Protocol header, or MBAP header, contains seven bytes including transaction ID for matching requests and responses, protocol ID which is always zero for Modbus, length field indicating the remaining message size, and unit ID for device addressing. Finally, the Protocol Data Unit contains the function code specifying the operation and the actual data to be transmitted.
Modbus TCP uses standardized function codes to perform different operations. Function code 01 reads coils which are digital outputs, 02 reads discrete inputs for digital input status, 03 reads holding registers containing read-write data, and 04 reads input registers for read-only data. Function codes 05 and 06 write to single coils and registers respectively. These functions enable comprehensive control and monitoring of industrial devices, with coils representing binary states and registers handling 16-bit numerical data.
Modbus TCP provides numerous advantages over traditional serial Modbus communication. It offers significantly higher data transmission speeds of 10 to 100 megabits per second, supports longer distances of over 100 meters per network segment, and leverages standard Ethernet infrastructure that is already present in most facilities. The protocol enables easy integration with existing networks, supports multiple simultaneous device connections without complex wiring, and reduces overall system complexity. These benefits make Modbus TCP the preferred choice for modern industrial automation, SCADA systems, factory automation, and building management applications.