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MQTT and Modbus are two protocols that are indispensable in the Internet of Things (IoT), covering 98% of the products on the market.
MQTT was released by IBM in 1999. In 2014, MQTT became a standard of OASIS (Organization for the Advancement of Structured Information Standards) and was later formally adopted as an international standard protocol by ISO/IEC 20922.
Modbus is a serial communication protocol developed by Modicon (Schneider) in 1979. It was originally designed for the company’s programmable logic controllers (PLCs).
Advantages and Disadvantages of MQTT
Advantages:
- • Based on a publish/subscribe model, it supports massive device access, making it inherently suitable for distributed systems (such as IoT cloud platforms).
- • Designed for unstable networks, it supports device reconnection and offline message caching (QoS mechanism).
- • A lightweight protocol (minimum message size is only 2 bytes), suitable for low bandwidth environments (wide area cellular networks).
- • Supports custom topics and JSON/binary data formats, accommodating complex business logic.
- • Provides three QoS levels (at most once, at least once, exactly once), balancing reliability and efficiency.
- • Supports TLS/SSL encryption and client authentication (username/password, certificates), suitable for open network environments.
Disadvantages:
- • Relies on an intermediary broker, increasing communication latency, making it unsuitable for millisecond-level real-time control.
- • Topics and message formats need to be defined by the user, lacking industry-wide standards, which may increase development costs.
- • Compared to Modbus, MQTT requires more computational resources to handle connection and subscription logic.
Advantages and Disadvantages of Modbus
Advantages:
- • Simple protocol structure, low implementation cost, suitable for rapid deployment in industrial automation.
- • Based on a master/slave architecture (client/server), with clear logic, suitable for control command transmission.
- • Suitable for industrial scenarios requiring low latency (such as PLC control), with high efficiency in direct reading and writing of device registers.
- • An established standard in the industrial field, almost all PLCs, sensors, and instruments support Modbus (RTU/TCP).
- • Requires minimal resources, allowing direct access to registers without going through the CPU.
Disadvantages:
- • The master/slave architecture limits the number of devices, making it difficult to support large-scale distributed systems; with many devices, wiring can be cumbersome (hand-in-hand connections also require long cables).
- • Only supports basic read/write operations (such as reading holding registers), unable to handle complex interactions.
- • Fixed data format (16-bit registers), requiring manual parsing of complex data types (such as floating-point numbers).
- • Lacks security mechanisms, relying on network isolation or firewall protection.
Conclusion
Modbus is more suitable for small-scale, real-time, and hardware resource-constrained industrial control scenarios, but projects with high security and scalability requirements should be chosen with caution. MQTT is suitable for large-scale, distributed devices, complex network environments, and cloud-integrated IoT applications, but requires additional investment in broker management and protocol design. These two protocols complement each other in certain aspects; for example, local cabinets can use Modbus for comprehensive data collection and then upload to the cloud via MQTT, emphasizing low cost while ensuring timeliness.
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