Click the public account below ⬇ to get more technical sharing and updates
In the Modbus TCP communication protocol, the Master and Slave are two fundamental roles in communication, each with distinct responsibilities.
🔹 Concept of Master
- Active initiator of communication: In the Modbus protocol, the Master is the only party that actively initiates requests.
- Sender of requests/instructions: The Master sends instructions to the Slave, such as reading and writing registers, reading and writing coils, etc.
- Handles multi-Slave communication logic: The Master can manage multiple Slaves simultaneously, each with a unique address (in Modbus TCP, this is IP + port).
- In Modbus TCP, the Master generally acts as a TCP client, actively connecting to the Slave’s server port (default is 502).
🔹 Concept of Slave
- Passive responder in communication: The Slave never actively communicates with the Master; it only responds upon receiving a request from the Master.
- Device that executes requests: This includes returning data (such as sensor values) or executing operations (such as writing to registers, turning on outputs, etc.).
- In Modbus TCP, the Slave generally acts as a TCP server, listening on port 502 for connections and requests from the Master.
🔹 Advantages of Host Software as Modbus Master
As host software (such as SCADA, PLC configuration tools, data acquisition clients, etc.), the most suitable role is the Master. The reasons are as follows:
✅ Reasonableness and Role Matching:
- The host typically needs to collect data, monitor, and control from field devices (Slaves), which is precisely the Master’s responsibility.
- Devices (such as PLCs, sensors) usually run Modbus TCP server (Slave) programs, waiting for the host to connect.
✅ Functional Implementation Suggestions:
- TCP client connection functionality: Ability to specify the target device IP and port (default 502) and establish a communication connection.
- Support for Modbus function codes: Implement common function codes (such as 0x01 read coils, 0x03 read holding registers, 0x06 write single register, 0x10 write multiple registers, etc.).
- Periodic data polling mechanism: Support timed reading of data from multiple Slaves and update the interface display or storage.
- Error handling and reconnection mechanism: Support timeout handling, automatic reconnection on disconnection, and other fault tolerance features.
- Logging and debugging tools: Facilitate development, debugging, and maintenance, recording requests, responses, and errors during communication.
🔹 Summary
| Item | Master | Slave |
|---|---|---|
| Active/Passive | Actively initiates requests | Passively waits for responses |
| Typical Role | Host (software) | PLC, instruments, sensors |
| Modbus TCP Role | TCP client | TCP server |
| Function | Data collection, control, monitoring | Data response, command execution |
Therefore, the host should act as the Master in Modbus TCP communication, implementing communication control logic and actively reading and controlling Slave devices, which is the most appropriate and standard architectural choice.
Consultation for Custom Services
Project evaluations are free of charge!
No deposit development!
Pay only when satisfied with the results!
#
Qt/C++ Technical Consulting and Problem Solving
#
Custom Development of Visual Algorithms/Software Projects
Welcome to followWeChat: CgsjeduQQ: 132237666247