Modbus RTU communication in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the Modbus RTU baud rate to 9600.

PLC Programming

modbus_config.baud_rate = [1]

Drag options to blanks, or click blank then click option'

A9600

B4800

C19200

D115200

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a baud rate that is too high or too low for the device.

Using a baud rate not supported by the hardware.

The standard baud rate for many Modbus RTU devices is 9600. Setting baud_rate to 9600 ensures proper communication speed.

2fill in blank

medium

Complete the code to set the Modbus RTU slave address to 10.

PLC Programming

modbus_config.slave_address = [1]

Drag options to blanks, or click blank then click option'

A10

B1

C100

D255

Attempts:

3 left

💡 Hint

Common Mistakes

Using an address outside the valid range.

Confusing master and slave addresses.

The slave address identifies the device on the Modbus network. Setting it to 10 targets the correct slave device.

3fill in blank

hard

Fix the error in the code to correctly read holding registers starting at address 40001.

PLC Programming

modbus_read(start_address=[1], quantity=5)

Drag options to blanks, or click blank then click option'

A40001

B40000

C39999

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using the human-readable address directly without adjusting for zero-based indexing.

Confusing coil and register addresses.

Modbus register addresses are zero-based in code. Address 40001 corresponds to 40000 in zero-based indexing.

4fill in blank

hard

Fill both blanks to configure parity and stop bits for Modbus RTU communication.

PLC Programming

modbus_config.parity = [1]
modbus_config.stop_bits = [2]

Drag options to blanks, or click blank then click option'

A"Even"

B"Odd"

C1

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Setting parity to 'Odd' when 'Even' is required.

Using 2 stop bits instead of 1, causing timing issues.

Modbus RTU commonly uses even parity and 1 stop bit for error checking and timing.

5fill in blank

hard

Fill all three blanks to create a dictionary mapping register addresses to their values, filtering only registers with values greater than 100.

PLC Programming

registers = {addr: value for addr, value in modbus_data.items() if value [1] [2]
filtered_registers = {k: v for k, v in registers.items() if k [3] 40010}

Drag options to blanks, or click blank then click option'

A>

B100

C<

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong comparison operators causing empty or incorrect results.

Mixing up register addresses and values in conditions.

The code filters registers with values greater than 100 and then selects those with addresses less than 40010.

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