Industrial automation rarely uses devices from a single brand. A plant may have a Siemens PLC, Delta VFD, Schneider energy meter, and an HMI from another company. The big question is: how do these machines talk to each other?
This is where Modbus communication becomes extremely important. In this guide, you’ll learn Modbus in simple, practical terms – exactly how it works and how to configure it in an S7-200 Smart PLC.
What is Modbus Communication?
Modbus is a universal industrial communication protocol used to exchange data between automation devices like PLCs, drives, meters, and SCADA systems.
In simple words:
Modbus is the common language of machines.
It works on a request-response system:
- One device asks for data
- The other device replies with data
Because of its simplicity and reliability, Modbus is one of the most widely used industrial communication protocols in PLC automation.
Master-Slave Principle (Core Concept)
Modbus communication always follows discipline. Only one device controls the conversation.
| Device | Role |
| PLC | Master (initiates request) |
| VFD / Energy Meter / Sensor | Slave (responds to request) |
The slave device never sends data on its own.
It only replies when the PLC asks.
Communication Flow
- PLC sends request
- Slave processes request
- Slave returns response
The slave device never sends data by itself – it only responds when the PLC asks. That’s why in most automation systems, PLC acts as Modbus Master.
Types of Modbus (RTU vs TCP)
Modbus works in two main formats. Understanding this is critical in industrial automation projects.
Modbus RTU
- Works on RS-485 serial cable
- Low cost
- Best for local field devices
- Used with VFDs, temperature controllers, energy meters
Modbus TCP/IP
- Works on Ethernet network
- Faster communication
- Used in SCADA & plant-wide monitoring
- Suitable for large automation systems
Modbus RTU vs Modbus TCP
| Feature | Modbus RTU | Modbus TCP |
| Speed | Medium | Fast |
| Cable | RS485 | Ethernet |
| Distance | Long (industrial field) | Network based |
| Cost | Low | Higher |
| Use Case | Field devices | SCADA & monitoring |
Wiring Method (Very Important)
RTU Wiring (RS-485)
You only need 2 wires:
- A+
- B-
Important rules:
- Use shielded cable (prevents industrial noise)
- Add 120-ohm termination resistor at last device
- Maintain polarity (A to A, B to B)
TCP Wiring
Very simple:
- Standard Ethernet cable
- Connect through network switch
Understanding Modbus Registers
To read or write data, you must understand Modbus memory areas.
| Register Type | Purpose | Example |
| Coils | Digital Output | Motor ON/OFF |
| Discrete Inputs | Digital Input | Limit switch status |
| Input Registers | Analog Read | Temperature value |
| Holding Registers | Read & Write | Speed setpoint |
In real projects, holding registers are used most.
Configuring Modbus in S7-200 Smart PLC (Step-by-Step)
Now comes the practical part – programming.
Step 1: Enable Modbus Port
In CPU settings:
- Select communication port
- Enable Modbus mode
- Set baud rate & parity
Parity:
- 0 = None
- 1 = Even
- 2 = Odd
Step 2: Add Modbus Control Block
Inside main program:
- Use Always ON bit
- Enable Modbus library control block
- Set timeout (example: 100 ms)
This converts the port into Modbus protocol mode.
Step 3: Create Modbus Message
Define parameters:
| Parameter | Meaning |
| Slave ID | Device number (example: energy meter = 1) |
| Function | Read = 0, Write = 1 |
| Register Address | Device register location |
| Count | Number of registers |
| PLC Address | Where data will store |
After execution:
- Done bit → success
- Error bit → communication fault
Reading Multiple Devices
You can read many devices on same network.
Example:
- Slave 1 → Energy meter
- Slave 2 → VFD
Just create multiple Modbus message blocks and assign different slave IDs.
Automatic Trigger Logic (Real Industry Method)
In real factories, we don’t manually trigger communication.
We use timers.
Timer-Based Data Polling
Example logic:
- 0–5 sec → Read Device 1
- 5–10 sec → Read Device 2
This ensures stable communication and prevents network collision.
This method is widely used in:
- SCADA data logging
- Energy monitoring systems
- Industrial IoT dashboards
Common Communication Errors
| Problem | Reason |
| No Data | Wrong slave ID |
| Communication Error | Baud rate mismatch |
| Garbage Values | Parity mismatch |
| Intermittent Fault | No termination resistor |
| Timeout | Wrong register address |
Always keep parameters same in both master and slave devices.
Where Modbus is Used (Real Industrial Applications)
- Energy meter monitoring
- VFD speed control
- Temperature monitoring system
- Water treatment plant automation
- SCADA data acquisition
- Building management systems
- PLC to SCADA integration
Final Thoughts
Modbus is not just another protocol – it is the backbone of industrial automation communication.
Once you understand:
- Master-Slave concept
- Register types
- Proper wiring
- Trigger logic
You can connect almost any industrial device to a PLC. By learning Modbus configuration in S7-200 Smart PLC, you can connect PLC with drives, meters, and SCADA systems easily – even across different brands.
For beginners in PLC programming, learning Modbus communication is a game-changer skill because real industrial projects always require machine-to-machine communication.
FAQ – Modbus Communication
What is Modbus in PLC?
It is a protocol that allows PLC to exchange data with other devices like VFD, meters, and sensors.
Difference between RTU and TCP?
RTU uses serial RS485 cable, TCP uses Ethernet network.
Why is RS485 used in industry?
Because it supports long distance communication and high noise immunity.
How does the PLC read energy meter data?
PLC sends a request to the meter register address and receives value in response.
