After reviewing the work, there was actually nothing especially difficult. It was just that in the DDD domain-driven development context of industrial automation, some technical terms in this field were still unfamiliar to me. Starting development directly came with a little confusion. The RS485 physical-layer protocol itself is quite simple. In fact, as an upper-computer application developer, I do not need to care too much about the physical-layer details. The key is how to use the Modbus industrial communication protocol according to the business requirement.
The latest term for the RS485 physical-layer protocol on Wikipedia is now EIA-485.
Why
The application I am currently developing needs to connect to an external hardware device. Data interaction between the application and the device happens through the RS485 protocol, which belongs to the physical layer in the OSI model.
Goal
The goal is to make data interaction between the application and hardware device work: complete data sending and receiving, then implement the related business features. The confusing part was data send/receive processing. The business feature development itself was much simpler.
Learning by Analogy + ChatGPT
As the master side of communication, the application uses a company-encapsulated SDK interface to read and write device registers through the ModbusRTU protocol.
As the slave side of communication, the device uses ModbusRTU classes from the Qt ecosystem to read and write device registers.
Both sides are essentially using the ModbusRTU industrial communication protocol.
In the whole data interaction development process, I only used function code 0x03, Read Holding Registers, and function code 0x10, Write Multiple Registers.
I also became familiar with how the Qt ecosystem uses the ModbusRTU protocol through QModbusRtuSerialSlave.
The rest was mainly learning through Q&A with ChatGPT and modifying code.
Notes from GPT Q&A and My Understanding
Modbus is a communication protocol widely used in industrial automation. It is mainly used for data exchange between industrial devices such as PLCs, sensors, frequency converters, instruments, and so on.
Industrial Properties of Modbus
| Property | Description |
|---|---|
| Design intent | Designed for industrial environments, emphasizing simplicity, reliability, and real-time behavior. |
| Typical scenarios | Factory automation, SCADA systems, power monitoring, building control, etc. |
| Hardware compatibility | Supports RS485, RS232, and Ethernet through Modbus TCP, adapting to industrial field wiring. |
| Protocol simplicity | Compact frame structure, easy for embedded devices to implement, suitable for resource-limited industrial controllers. |
| Master-slave architecture | Suitable for centralized monitoring in industrial control, such as a PLC as master and sensors/actuators as slaves. |
Industrial Application Examples of Modbus
- PLC communication: Siemens, Schneider, and other PLCs communicate with peripheral devices through Modbus.
- Sensor data acquisition: temperature and pressure sensors report data through Modbus RTU.
- Frequency converter control: control motor speed, such as ABB converters supporting Modbus.
- Power monitoring: electric meters use Modbus to transmit voltage and current data.
- HMI-device interaction: touch screens access PLC data through Modbus TCP.
Why Is Modbus Popular in Industry?
- Standardized: an open protocol without copyright restrictions, widely supported by vendors.
- Low overhead: simple protocol with low hardware resource requirements, suitable for low-cost embedded devices.
- Flexible: supports multiple physical layers such as RS485, RS232, and TCP.
- Mature and stable: introduced in 1979 and validated in industrial sites for a long time.
Limitations of Modbus
- No built-in security: no encryption or authentication, so it needs physical isolation or firewall protection.
- Limited real-time performance: slower than modern industrial protocols such as EtherCAT and PROFINET.
- Data size limits: the number of registers per read/write is limited, such as a maximum of 123 registers for Modbus RTU.
Comparing Modbus with Other Industrial Protocols
| Protocol | Layer | Characteristics | Suitable scenarios |
|---|---|---|---|
| Modbus | Application layer | Simple, general-purpose, low cost | Small and medium industrial control systems |
| PROFINET | Real-time Ethernet | High speed and deterministic latency | High-performance automation, such as automotive manufacturing |
| EtherCAT | Real-time Ethernet | Distributed clocks and ultra-low latency | Precision motion control, such as robotics |
| CANopen | Data link layer | Strong anti-interference and multi-master architecture | Vehicles and engineering machinery |
End
I have now entered one more knowledge domain. When I run into similar problems later, they should be much easier to handle.