Modbus Protocol - A Python Beginner's Summary
I work in Tokyo as a B2B sales rep for wastewater treatment equipment. Recently, there's been growing demand not just for automated equipment itself, but also for remote monitoring and IoT integration of facilities.
How can I respond to these needs? This question is what motivated me to start learning Python, AWS, AI, and ML.
Today, I want to leave some notes about the Modbus Protocol, which is the foundation of Python's Pymodbus library, from a beginner's perspective.
What is the Modbus Protocol?
The Modbus Protocol is a set of communication rules for industrial machines to talk to each other. Originally developed by Modicon (a PLC manufacturer) in 1979.
The protocol specifications are publicly available and anyone can use it commercially for free.
Due to its simplicity, many industrial devices support the Modbus Protocol.
How Modbus Protocol Communication Works
Modbus Protocol uses a master/slave communication method.
Master (Client): PC, monitoring systems
Slave (Server): Sensors, actuators, PLCs
The actual process works like this:
- Master sends a command (request)
- Only the specified slave executes that command
- Slave sends back the result (response)
- Master receives the result
Internal Structure of Modbus-Compatible Devices
1. Memory Space (Data Areas)
Modbus Protocol-compatible devices have 4 types of memory spaces for storing information, just like PC memory. These are called data areas.
- Coil: Stores digital values (ON/OFF or True/False), read/write access
- Discrete Input: Stores digital values (ON/OFF or True/False), read-only access
- Input Register: Stores analog values (numerical data), read-only access
- Holding Register: Stores analog values (numerical data), read/write access
| Item | Digital Values | Analog Values |
|---|---|---|
| Data Nature | Discrete (ON/OFF) | Continuous (numerical) |
| Representation | True/False, 1/0 | 0~65535 numbers |
| Data Size | 1 bit | 16 bits |
| Use Cases | Switches, relays, LEDs | Temperature, pressure, flow, voltage |
| Modbus Data Areas | • Coil • Discrete Input |
• Input Register • Holding Register |
Key Point
Each manufacturer and model stores different things in different areas, so you must always check the manual.
Example: Temperature Controller
| Data Area | Data Type | Access | Stored Information |
|---|---|---|---|
| Coil | Digital | Read/Write | Heater control, alarm reset |
| Discrete Input | Digital | Read-only | Alarm status, operation status |
| Input Register | Analog | Read-only | Current temperature, sensor values |
| Holding Register | Analog | Read/Write | Set temperature, control parameters |
2. Types of Modbus Communication
| Type | Communication Method | Data Format | Connection | Features |
|---|---|---|---|---|
| Modbus RTU | Serial communication | Binary | RS-485 cable | • Most common • Fast & efficient • Compact data |
| Modbus ASCII | Serial communication | ASCII characters | RS-485 cable | • Human-readable format • Easy to debug • Double data volume |
| Modbus TCP/IP | Ethernet | Binary | LAN cable | • LAN cable connection • Internet capable • High-speed, long-distance |
Modern PLCs (Mitsubishi Electric, Omron, Keyence, etc.) have standard support for Modbus TCP/IP communication, making it mainstream for new systems.
Summary
I didn't get to write much code today, but I gained some fundamental knowledge about the technology that supports IoT.
Next time, I want to try writing actual Modbus communication code using the Pymodbus library.
See you tomorrow! 👋
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