Modbus RTU communication in PLC Programming - Mini Project: Build & Apply

Modbus RTU Communication Setup

📖 Scenario: You are setting up a simple Modbus RTU communication between a PLC and a temperature sensor. The PLC will read temperature data from the sensor using Modbus RTU protocol.

🎯 Goal: Build a basic Modbus RTU communication script that reads a temperature value from a sensor and displays it.

📋 What You'll Learn

Create a variable for the Modbus slave address

Set up the Modbus RTU communication parameters

Read the temperature register from the sensor

Display the temperature value

💡 Why This Matters

🌍 Real World

Modbus RTU is widely used in industrial automation to connect PLCs with sensors and devices over serial communication.

💼 Career

Understanding Modbus RTU communication is essential for automation engineers and technicians working with PLCs and industrial networks.

1

Set the Modbus Slave Address

Create a variable called slave_address and set it to 1 to represent the Modbus device address.

PLC Programming

# Create a variable called slave_address and set it to 1
# Your code here

Hint

The slave address is the ID of the device you want to communicate with. Usually, it is a number like 1.

2

Configure Modbus RTU Parameters

Create a dictionary called modbus_config with keys baudrate set to 9600, parity set to 'N', and stopbits set to 1.

PLC Programming

slave_address = 1
# Create a dictionary called modbus_config with baudrate=9600, parity='N', stopbits=1
# Your code here

Hint

These settings define how the PLC talks to the sensor: speed, parity check, and stop bits.

3

Read Temperature Register

Create a variable called temperature_register and set it to 100. Then create a variable called temperature_value and simulate reading by setting it to 25.5.

PLC Programming

slave_address = 1
modbus_config = {'baudrate': 9600, 'parity': 'N', 'stopbits': 1}
# Set temperature_register to 100
# Set temperature_value to 25.5 to simulate reading
# Your code here

Hint

The register number 100 is where the temperature is stored. We simulate reading 25.5 degrees.

4

Display the Temperature Value

Write a print statement to display the text "Temperature: 25.5 °C" using the variable temperature_value.

PLC Programming

slave_address = 1
modbus_config = {'baudrate': 9600, 'parity': 'N', 'stopbits': 1}
temperature_register = 100
temperature_value = 25.5
# Print the temperature value in the format: Temperature: 25.5 °C
# Your code here

Hint

Use an f-string to insert the temperature_value into the print statement.

Practice

1. What is the main purpose of Modbus RTU communication in PLC programming?

easy

A. To exchange data between devices over serial communication

B. To create graphical user interfaces

C. To compile PLC programs

D. To store data in cloud servers

Solution

1. Step 1: Understand Modbus RTU basics

   Modbus RTU is a protocol used for serial communication between devices like PLCs and sensors.

2. Step 2: Identify the main function

   Its main function is to exchange data by sending requests and responses over serial lines.

3. Final Answer:

   To exchange data between devices over serial communication -> Option A

4. Quick Check:

   Modbus RTU = Data exchange over serial [OK]

Hint: Modbus RTU is about device data exchange via serial lines [OK]

Common Mistakes:

• Confusing Modbus RTU with programming languages
• Thinking it creates user interfaces
• Assuming it stores data remotely

2. Which of the following is the correct format for a Modbus RTU request frame?

easy

A. CRC + Device Address + Function Code + Data

B. Device Address + Function Code + Data + CRC

C. Data + Device Address + Function Code + CRC

D. Function Code + Device Address + Data + CRC

Solution

1. Step 1: Recall Modbus RTU frame structure

   The request frame starts with the device address, followed by the function code, then data, and ends with CRC for error checking.

2. Step 2: Match the correct sequence

   Device Address + Function Code + Data + CRC matches this exact order: Device Address + Function Code + Data + CRC.

3. Final Answer:

   Device Address + Function Code + Data + CRC -> Option B

4. Quick Check:

   Frame order = Address, Function, Data, CRC [OK]

Hint: Remember: Address first, then function, data, and CRC last [OK]

Common Mistakes:

• Mixing up the order of function code and device address
• Placing CRC at the start instead of the end
• Confusing data position in the frame

3. Given this Modbus RTU request frame in hex: 01 03 00 0A 00 02 C5 CD, what does the function code 03 represent?

medium

A. Write Multiple Registers

B. Write Single Coil

C. Read Input Status

D. Read Holding Registers

Solution

1. Step 1: Identify the function code in the frame

   The second byte in the frame is 03, which is the function code.

2. Step 2: Recall Modbus function codes

   Function code 03 is used to read holding registers in Modbus RTU communication.

3. Final Answer:

   Read Holding Registers -> Option D

4. Quick Check:

   Function code 03 = Read Holding Registers [OK]

Hint: Function code 03 always reads holding registers [OK]

Common Mistakes:

• Confusing function code 03 with write operations
• Mixing read input status (code 02) with 03
• Ignoring the function code position in the frame

4. You wrote a Modbus RTU request but the PLC does not respond. Which of these is the most likely error in your request frame?

medium

A. Using ASCII instead of RTU mode

B. Sending data without device address

C. Incorrect CRC value

D. Using function code 99

Solution

1. Step 1: Check common communication errors

   Incorrect CRC causes the PLC to reject the frame because it detects data corruption.

2. Step 2: Evaluate other options

   Using ASCII mode or missing device address would cause errors, but CRC error is the most common cause of no response. Function code 99 is invalid but usually triggers an error response, not silence.

3. Final Answer:

   Incorrect CRC value -> Option C

4. Quick Check:

   Bad CRC = No response from PLC [OK]

Hint: Always verify CRC to avoid silent communication failures [OK]

Common Mistakes:

• Ignoring CRC errors
• Assuming wrong function code causes no response
• Confusing ASCII and RTU modes

5. You want to read 4 consecutive holding registers starting at address 100 from a device with address 5 using Modbus RTU. Which request frame (in hex) correctly represents this?

hard

A. 05 03 00 64 00 04 45 C9

B. 05 04 00 64 00 04 45 C9

C. 01 03 00 64 00 04 45 C9

D. 05 03 00 65 00 04 45 C9

Solution

1. Step 1: Understand the frame components

   Device address is 05, function code for reading holding registers is 03, starting address 100 in hex is 00 64, number of registers to read is 00 04, followed by CRC.

2. Step 2: Check each option

   05 03 00 64 00 04 45 C9 matches all these correctly. 05 04 00 64 00 04 45 C9 uses function code 04 (read input registers), 01 03 00 64 00 04 45 C9 uses device address 01, and 05 03 00 65 00 04 45 C9 starts at address 101 (00 65).

3. Final Answer:

   05 03 00 64 00 04 45 C9 -> Option A

4. Quick Check:

   Address 5 + Func 03 + Start 100 + Count 4 = 05 03 00 64 00 04 45 C9 [OK]

Hint: Match device address, function code, start address, and count exactly [OK]

Common Mistakes:

• Using wrong function code for holding registers
• Incorrect device address
• Off-by-one error in start address