The Modbus protocol is widely used in the field of industrial automation. Its main derivative forms are Modbus RTU (Remote Terminal Unit) and Modbus TCP (Transmission Control Protocol), which are used for serial bus and Ethernet transmission respectively. Although they both belong to the Modbus family, there are significant differences in the physical layer, data encapsulation, communication reliability, etc.
Our company has a rich and complete range of Modbus devices. For Modbus RTU, you can refer to the MA01 series; for Modbus TCP, refer to the serial server series; for the integration of Modbus RTU and TCP, refer to the distributed IO series.
Physical Layer and Transmission Media
Modbus RTU
It relies on RS - 485 (or RS - 232) serial communication, with a bus topology structure. The typical baud rate ranges from 9600 to 115200 bps.
The RS - 485 bus can achieve half - duplex multi - point communication, allowing a maximum of 32 devices to be connected in parallel. Under ideal conditions, the bus length can reach about 1200 meters.Modbus TCP
It operates on the Ethernet physical layer and transmits data based on the TCP/IP protocol stack. Industrial Ethernet switches, routers, or wireless local area networks can be used to improve network coverage and transmission speed.
Theoretically, it supports speeds of 100 Mbps (Fast Ethernet), 1 Gbps (Gigabit Ethernet), or even higher. The transmission distance is limited by network devices and link architectures. In a local area network environment, it can cover several kilometers, and cross - network segments can achieve global remote connections.
Protocol Frame Structure and Encapsulation Method
Modbus RTU Frame Format
It consists of "Silent interval (at least 3.5 character times) → Slave address (1 byte) → Function code (1 byte) → Data field (N bytes) → CRC check (2 bytes)".
CRC (Cyclic Redundancy Check) is used to detect errors in the data transmission process. If the idle time between frames exceeds the specified threshold, it will be regarded as the start of a new frame.
Modbus TCP Frame Format
The Modbus protocol data is embedded in the TCP message. The typical structure is as follows:
MBAP (Modbus Application Protocol Header, 7 bytes): Transaction identifier (2 bytes), Protocol identifier (2 bytes, fixed at 0), Length field (2 bytes, indicating the number of subsequent bytes), Unit identifier (1 byte, for compatibility with traditional slave addresses).
PDU (Protocol Data Unit, N bytes): It contains the function code (1 byte) and the data field (N - 1 bytes).
Since TCP itself has a check and re - transmission mechanism, the Modbus TCP frame does not require an additional CRC check. The frame boundary is managed by TCP flow control and does not rely on the silent interval.
The MBAP header increases the addressing and transaction processing capabilities, enabling multiple clients to distinguish their respective requests and responses when concurrently accessing the same slave device.
Communication Reliability and Real - Time Performance
Modbus RTU
Serial bus communication has low latency, and the protocol's own delay is controllable. However, the bus topology structure is prone to signal attenuation and frame errors due to too many nodes or poor wiring quality.
Thanks to the CRC check mechanism, it can identify and discard erroneous data to a certain extent. Usually, in a field with a limited number of devices and low bus load, the performance is stable.
It is very sensitive to the idle time between frames. If there is "jitter" between the master and the slave or poor cable contact, it may lead to communication interruption or re - transmission.Modbus TCP
Based on Ethernet and the TCP protocol, the congestion control and re - transmission mechanisms can improve data integrity. However, this also introduces additional handshake and confirmation delays, resulting in a slightly higher overall delay than RTU.
In an environment with a reasonable network architecture and properly configured switches and routers, the packet loss rate is extremely low, and re - transmission is possible, achieving high reliability. It is particularly suitable for large - scale concurrent access and remote monitoring.