Modbus is a widely used communication protocol in industrial automation and control systems. It comes in different variants, with Modbus RTU and Modbus TCP being two of the most common versions. Here are the key differences between Modbus RTU and Modbus TCP:

1. Physical Layer:

   • Modbus RTU: Modbus RTU (Remote Terminal Unit) uses serial communication over RS-232 or RS-485 interfaces. It is a master-slave protocol where one master device communicates with multiple slave devices over a serial bus.
   • Modbus TCP: Modbus TCP (Transmission Control Protocol) uses Ethernet as the physical layer and is based on the TCP/IP network protocol stack. It operates over standard Ethernet networks, including wired and wireless connections.

2. Data Transmission:

   • Modbus RTU: In Modbus RTU, data is transmitted in binary format over a serial link. It uses a simple framing protocol where each data frame consists of address, function code, data, error checking, and a frame delimiter.
   • Modbus TCP: In Modbus TCP, data is transmitted as messages encapsulated in TCP/IP packets. It uses TCP for reliable, connection-oriented communication, allowing for error detection, retransmission, and flow control.

3. Speed and Throughput:

   • Modbus RTU: Modbus RTU typically operates at lower baud rates, ranging from 1200 to 115200 bits per second (bps), depending on the network configuration and distance. It has lower throughput compared to Modbus TCP.
   • Modbus TCP: Modbus TCP operates over Ethernet networks, which can support higher data rates and throughput compared to serial communication. It is capable of reaching speeds of up to 100 Mbps or more, depending on the network infrastructure.

4. Topology:

   • Modbus RTU: Modbus RTU networks typically use a daisy-chain or multi-drop topology, where multiple devices are connected in series or parallel to a single communication bus.
   • Modbus TCP: Modbus TCP networks use standard Ethernet infrastructure, including switches, routers, and hubs. They can support various network topologies, including star, ring, and mesh configurations.

5. Addressing:

   • Modbus RTU: Modbus RTU devices are typically addressed using a unique slave address ranging from 1 to 247. The master device communicates with individual slave devices using their respective addresses.
   • Modbus TCP: Modbus TCP devices are identified using their IP addresses on the Ethernet network. Each device has a unique IP address, allowing for direct communication between devices without the need for addressing schemes used in Modbus RTU.

6. Compatibility and Interoperability:

   • Modbus RTU: Modbus RTU is widely supported by a wide range of industrial devices, controllers, and sensors. It is commonly used in legacy systems and applications where serial communication is preferred.
   • Modbus TCP: Modbus TCP is becoming increasingly popular due to its compatibility with Ethernet networks and modern industrial automation systems. It offers greater flexibility, scalability, and interoperability compared to Modbus RTU.

In summary, while both Modbus RTU and Modbus TCP serve similar purposes in industrial communication, they differ in terms of physical layer, data transmission, speed, topology, addressing, and compatibility. Modbus RTU is suited for serial communication over RS-232 or RS-485 interfaces, while Modbus TCP operates over Ethernet networks using TCP/IP protocol stack. The choice between Modbus RTU and Modbus TCP depends on factors such as network infrastructure, communication requirements, and device compatibility.