Modbus Modbus is a serial communication protocol used to connect industrial automation equipment. It was initially developed by Modicon in 1979 and has become one of the universal communication standards widely used in industrial automation scenarios. Modbus adopts a master-slave mode and supports multiple transmission modes, including serial (RS-²³²⁄₄₈₅) and Ethernet version (TCP/IP). It is mainly used for monitoring and controlling automation equipment such as sensors, motors, and PLCs. It enables data exchange between devices and sending control commands, allowing devices to work together in coordination.

With the arrival of Industry 4.0, the demand for intelligence, automation, and digitization in manufacturing industries is increasing. In this context, the Modbus protocol faces some challenges. Firstly, there are significant security issues with Modbus protocol due to its simple communication method that makes it susceptible to hacking attacks and data tampering. Secondly, compared with modern industrial communication standards in real-time performance and bandwidth utilization, the Modbus protocol appears less excellent. Especially when large-scale device networking occurs, traditional serial communication methods cannot meet requirements.

In addition, there are certain limitations of the Modbus protocol regarding multi-layer architecture and cloud platform applications. It can only perform point-to-point communication and does not support distributed systems or cloud computing platforms very well.

MQTT MQTT is a lightweight message transfer protocol based on a publish/subscribe model built on top of TCP/IP protocol. It was published by IBM in 1999 and became an officially approved communication standard by OASIS on October 29th, 2014. MQTT has been widely used in fields such as IoT, mobile internet, smart hardware, connected cars, smart cities, remote medical care, electricity industry as well as oil & energy industries.

The following table shows an analysis of the differences between different versions of mainstream MQTT protocols. Version v1 v3.1 (v3) v3.1.1 (v4) v5 Time 1999 2013 2014 2019 Security - Add username and password in the connect message - 1) Add user attribute field indicating message source in the connect and publish messages.

1. Add authorization, security communication mechanisms besides authentication. Reliability - Add return codes in connack message; add error codes indicating whether the subscription is prohibited in suback message. 1) Add a mechanism to prevent the expiration of messages to prevent erroneous instructions.
2. Support client to obtain broker capabilities. Network/Computational Resource Saving - - 1) Add Session Present flag indicating whether broker has an earlier session in connack message.
3. Anonymous clients that only publish without subscribing.
4. Micro-restricted clients instantly publishing messages without waiting for responses. 1) Broadcast publish topic with zero length.
5. Add disconnect message from broker to client.

Overall, MQTT V5 has improved its performance, reliability, and scalability compared with previous versions. Its improvements make MQTT more suitable for large-scale highly distributed systems.

EMQX Cloud is an MQTT middleware product designed for the IoT field launched by EMQ company. As the world’s first fully managed MQTT 5.0 messaging cloud service, EMQX Cloud provides one-stop operation and maintenance agent services as well as unique isolated environments for MQTT messaging services.

The following figure shows the data acquisition model provided by EMQX Cloud.