Modbus TCP/IP in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this Modbus TCP/IP read command simulation?

Consider a script that simulates reading holding registers from a Modbus TCP/IP device. What will be the output of the following code snippet?

PLC Programming

registers = {100: 1234, 101: 5678, 102: 9012}
address = 101
value = registers.get(address, None)
print(f"Register {address} value: {value}")

A Register 101 value: 5678

B Register 101 value: None

C Register 100 value: 1234

D KeyError: 101

💡 Hint

Look at how the dictionary get method works with the given address.

The dictionary 'registers' contains the key 101 with value 5678. Using get(address, None) returns 5678, so the output prints that value.

🧠 Conceptual

intermediate

Which Modbus TCP/IP function code is used to write a single coil?

In Modbus TCP/IP communication, which function code is used to write a single coil (digital output)?

A 0x10

B 0x05

C 0x03

D 0x01

💡 Hint

Function codes are hexadecimal numbers representing specific actions.

Function code 0x05 is used to write a single coil in Modbus TCP/IP.

🔧 Debug

advanced

Identify the error in this Modbus TCP/IP client connection code snippet

What error will this Python code raise when trying to connect to a Modbus TCP/IP server?

PLC Programming

from pymodbus.client.sync import ModbusTcpClient
client = ModbusTcpClient('192.168.1.100')
client.connect()
result = client.read_holding_registers(1, 2)
print(result.registers)
client.close()

A AttributeError: 'NoneType' object has no attribute 'registers'

B ConnectionRefusedError: [Errno 111] Connection refused

C SyntaxError: invalid syntax

D TypeError: read_holding_registers() missing 1 required positional argument

💡 Hint

Check if the read_holding_registers call returns None on failure.

If the connection fails or the read fails, result is None, so accessing result.registers raises AttributeError.

🚀 Application

advanced

How many registers are read by this Modbus TCP/IP read command?

Given the following Modbus TCP/IP read command, how many registers will be read?

PLC Programming

read_holding_registers(address=400, count=10)

A 410 registers

B 400 registers

C 1 register

D 10 registers

💡 Hint

The 'count' parameter specifies how many registers to read.

The 'count' parameter is 10, so 10 registers starting at address 400 are read.

📝 Syntax

expert

What error does this Modbus TCP/IP write command produce?

What error will this Python code produce when trying to write multiple registers?

PLC Programming

from pymodbus.client.sync import ModbusTcpClient
client = ModbusTcpClient('192.168.1.100')
client.connect()
result = client.write_registers(10, [100, 200, 300])
print(result)
client.close()

A AttributeError: 'NoneType' object has no attribute 'isError'

B TypeError: write_registers() missing 1 required positional argument

C No error, prints <WriteMultipleRegistersResponse>

D ConnectionError: Failed to connect to server

💡 Hint

Check the pymodbus write_registers method signature and return type.

If connection succeeds, write_registers returns a response object that prints normally without error.

Practice

1. What is the main purpose of Modbus TCP/IP in PLC programming?

easy

A. To convert analog signals to digital

B. To program PLCs using ladder logic

C. To allow devices to communicate over Ethernet using simple commands

D. To create graphical user interfaces for PLCs

Solution

1. Step 1: Understand Modbus TCP/IP function

   Modbus TCP/IP is a communication protocol that enables devices to exchange data over Ethernet networks.

2. Step 2: Identify the main use

   It uses simple commands to read and write data between devices, facilitating communication.

3. Final Answer:

   To allow devices to communicate over Ethernet using simple commands -> Option C

4. Quick Check:

   Modbus TCP/IP = Ethernet communication protocol [OK]

Hint: Modbus TCP/IP is about device communication over Ethernet [OK]

Common Mistakes:

• Confusing Modbus TCP/IP with PLC programming languages
• Thinking it converts signals instead of communicating data
• Assuming it creates user interfaces

2. Which of the following is the correct syntax to read holding registers using Modbus TCP/IP function code in a PLC script?

easy

A. ReadHoldingRegisters(address, quantity)

B. WriteCoils(address, quantity)

C. ReadInputStatus(address, quantity)

D. WriteHoldingRegisters(address, quantity)

Solution

1. Step 1: Identify function codes for reading registers

   Modbus function code 03 reads holding registers, typically called ReadHoldingRegisters.

2. Step 2: Match syntax to function

   ReadHoldingRegisters(address, quantity) correctly represents reading holding registers by specifying start address and number of registers.

3. Final Answer:

   ReadHoldingRegisters(address, quantity) -> Option A

4. Quick Check:

   Read holding registers = ReadHoldingRegisters() [OK]

Hint: Read holding registers uses ReadHoldingRegisters() function [OK]

Common Mistakes:

• Using Write functions instead of Read
• Confusing coils with registers
• Using incorrect function names

3. Given the following PLC script snippet using Modbus TCP/IP:

response = ReadCoils(10, 5)
print(response)

What is the expected output if coils 10 to 14 are ON, OFF, ON, OFF, ON respectively?

medium

A. [1, 0, 1, 0, 1]

B. Error: Invalid function

C. [ON, OFF, ON, OFF, ON]

D. [True, False, True, False, True]

Solution

1. Step 1: Understand ReadCoils function output

   ReadCoils returns a list of boolean values representing coil states: True for ON, False for OFF.

2. Step 2: Map coil states to boolean list

   Coils 10 to 14 states ON, OFF, ON, OFF, ON map to [True, False, True, False, True].

3. Final Answer:

   [True, False, True, False, True] -> Option D

4. Quick Check:

   Coil states as booleans = [True, False, True, False, True] [OK]

Hint: ReadCoils returns booleans True/False, not strings or numbers [OK]

Common Mistakes:

• Expecting numeric 1/0 instead of True/False
• Using string 'ON'/'OFF' instead of booleans
• Assuming function returns error

4. A PLC script using Modbus TCP/IP to write to coils is:

WriteCoils(5, [True, False, True])

Which of the following correctly describes the WriteCoils function?

medium

A. The function WriteCoils requires a starting address and a list of boolean values

B. The list should contain integers 1 and 0 instead of booleans

C. The function WriteCoils only writes single coil, not multiple

D. The address 5 is out of range for coils

Solution

1. Step 1: Check WriteCoils function parameters

   WriteCoils expects a starting address and a list of boolean values to write multiple coils.

2. Step 2: Verify parameter usage

   The given call correctly provides the starting address and list of boolean values.

3. Step 3: Analyze options

   The accurate statement is that the function requires a starting address and a list of boolean values. Other statements are incorrect: address range is not specified as invalid, booleans are appropriate (not integers), and the function supports multiple coils.

4. Final Answer:

   The function WriteCoils requires a starting address and a list of boolean values -> Option A

5. Quick Check:

   WriteCoils(address, bool list) is correct syntax [OK]

Hint: WriteCoils needs address plus boolean list, not integers [OK]

Common Mistakes:

• Using integers instead of booleans
• Assuming WriteCoils writes only one coil
• Ignoring address range validity

5. You want to read 10 holding registers starting at address 100 and write the sum of their values to register 200 using Modbus TCP/IP in a PLC script. Which approach correctly combines reading, summing, and writing?

hard

A. values = ReadCoils(100, 10)
sum_val = sum(values)
WriteHoldingRegisters(200, sum_val)

B. values = ReadHoldingRegisters(100, 10)
sum_val = sum(values)
WriteHoldingRegisters(200, [sum_val])

C. values = ReadHoldingRegisters(100, 10)
sum_val = sum(values)
WriteCoils(200, sum_val)

D. values = ReadHoldingRegisters(100, 10)
sum_val = sum(values)
WriteHoldingRegisters(200, sum_val[0])

Solution

1. Step 1: Read holding registers correctly

   Use ReadHoldingRegisters(100, 10) to get 10 registers starting at 100.

2. Step 2: Sum the values and write properly

   Sum the list with sum(values), then write the sum as a list [sum_val] to register 200 using WriteHoldingRegisters.

3. Step 3: Check other options for errors

   values = ReadCoils(100, 10)
   sum_val = sum(values)
   WriteHoldingRegisters(200, sum_val)

   reads coils instead of registers;

   values = ReadHoldingRegisters(100, 10)
   sum_val = sum(values)
   WriteCoils(200, sum_val)

   writes to coils instead of registers;

   values = ReadHoldingRegisters(100, 10)
   sum_val = sum(values)
   WriteHoldingRegisters(200, sum_val[0])

   incorrectly indexes sum_val which is an int, causing error.

4. Final Answer:

   values = ReadHoldingRegisters(100, 10) sum_val = sum(values) WriteHoldingRegisters(200, [sum_val]) -> Option B

5. Quick Check:

   Read registers, sum list, write list with sum [OK]

Hint: Read registers, sum list, write sum as single-item list [OK]

Common Mistakes:

• Reading coils instead of registers
• Writing to coils instead of registers
• Passing integer instead of list to WriteHoldingRegisters