Modbus RTU communication in PLC Programming

Introduction

Modbus RTU helps devices like sensors and controllers talk to each other using simple messages over wires.

You want to read temperature or pressure from a sensor connected to your controller.

You need to control a motor or valve remotely using a PLC.

You want to collect data from multiple devices on the same communication line.

You need a reliable way to send commands and get responses in an industrial setup.

Syntax

PLC Programming

Function Block ModbusRTU(
  Input: Address, FunctionCode, RegisterAddress, DataToWrite
) -> Output: DataRead, Status

// Typical steps:
// 1. Set device address
// 2. Choose function code (e.g., read or write)
// 3. Specify register address
// 4. Provide data if writing
// 5. Receive data and status

Modbus RTU uses a serial line with a specific baud rate and parity settings.

Function codes define the action, like reading or writing registers.

Examples

Read holding registers starting at address 100 from device 1.

PLC Programming

ModbusRTU(Address=1, FunctionCode=3, RegisterAddress=100, DataToWrite=0) -> DataRead, Status

Write the value 1234 to register 200 on device 2.

PLC Programming

ModbusRTU(Address=2, FunctionCode=6, RegisterAddress=200, DataToWrite=1234) -> DataRead, Status

Sample Program

This program reads data from a sensor using Modbus RTU. It checks if the read was successful before using the data.

PLC Programming

// Simple Modbus RTU read example
// Read temperature from sensor at address 1, register 300

VAR
  DataRead: INT;
  Status: BOOL;
END_VAR

// Call ModbusRTU function block
ModbusRTU(Address:=1, FunctionCode:=3, RegisterAddress:=300, DataToWrite:=0, DataRead=>DataRead, Status=>Status);

IF Status THEN
  // Print or use the read data
  // For example, display temperature
  // (In real PLC, this might update a display or variable)
END_IF

Important Notes

Always check the Status output to know if communication succeeded.

Set the correct baud rate and parity in your PLC communication settings before using Modbus RTU.

Use the right function code for your task: 3 for reading holding registers, 6 for writing single registers.

Summary

Modbus RTU is a simple way for devices to exchange data over serial lines.

You use function blocks with parameters like device address and register to read or write data.

Always verify communication status to handle errors.

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