Differences among the three transmission modes of MODBUS protocol
In 1979, Modicon first introduced the serial Modbus standard. Later, due to the widespread adoption of networks and the need for higher transmission speeds, the Modbus standard based on TCP networks was established in 1997.
Therefore, there are two main transmission modes: those based on serial links and those based on Ethernet TCP/IP. However, it is customary to categorize them into three types: MODBUS-RTU, MODBUS-TCP, and MODBUS-ASCII.
These three primary transmission modes exhibit significant differences in data encoding, physical layer, efficiency, and application scenarios.
Detailed explanation:
1、MODBUS-RTU (Remote Terminal Unit)
Features: Utilizing binary encoding, it boasts the highest transmission efficiency and is the most commonly used mode in industrial settings. Its frame structure is concise, separating messages through silent intervals between data frames, and employs a 16-bit cyclic redundancy check (CRC) to ensure data integrity.
Applicable Scenario: It is suitable for local industrial control systems with short communication distances (usually not exceeding 1200 meters) and a limited number of devices, such as communication between PLCs and temperature sensors, frequency converters, and other devices in factory workshops.
2、MODBUS-TCP (Transmission Control Protocol)
Features: The MODBUS protocol is encapsulated within the TCP/IP protocol stack, enabling communication via Ethernet. It eliminates the address and CRC check found in the RTU mode and incorporates an MBAP (MODBUS Application Protocol) packet header, which contains information such as transaction identifiers and protocol identifiers, facilitating packet routing and identification within the network. The traditional slave address has been replaced by an IP address.
Applicable Scenario: It is suitable for large-scale systems that require remote monitoring, control, and networked management, such as cross-plant SCADA systems, energy management for intelligent buildings, and integration with enterprise IT networks.
3、MODBUS-ASCII (American Standard Code for Information Interchange)
Features: Data is encoded using ASCII characters, with each byte split into two ASCII characters for transmission. Messages start with a colon ":" and end with a carriage return/line feed (CR/LF), employing longitudinal redundancy check (LRC). Its greatest advantage is good data readability, allowing plaintext data to be directly viewed on the terminal, facilitating manual debugging.
Applicable scenarios: Due to its low transmission efficiency (with a rate typically not exceeding 9600 bps), it is primarily used in scenarios such as debugging, diagnosis, and teaching, where speed is not critical but direct viewing of data content is required.
