Modbus TCP/IP in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the Modbus TCP/IP port number.

PLC Programming

modbus_port = [1]

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

A23

B80

C502

D443

Attempts:

3 left

💡 Hint

Common Mistakes

Using HTTP port 80 instead of Modbus port 502.

Confusing Telnet port 23 with Modbus port.

The standard port number for Modbus TCP/IP communication is 502.

2fill in blank

medium

Complete the code to create a Modbus TCP client connection.

PLC Programming

client = ModbusTcpClient('[1]')

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

A192.168.1.100

B255.255.255.0

Clocalhost

D10.0.0.256

Attempts:

3 left

💡 Hint

Common Mistakes

Using an invalid IP address like 10.0.0.256 (256 is out of range).

Using a subnet mask instead of an IP address.

The client must connect to a valid IP address of the Modbus server; 192.168.1.100 is a valid private IP.

3fill in blank

hard

Fix the error in the code to read holding registers starting at address 10.

PLC Programming

result = client.read_holding_registers([1], 5)

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

A0x10

B'10'

Cten

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string '10' instead of integer 10.

Using a word 'ten' which is invalid.

The address must be an integer; 10 is correct. Strings or words cause errors.

4fill in blank

hard

Fill both blanks to write values [100, 200] to holding registers starting at address 20.

PLC Programming

client.write_registers([1], [2])

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

A20

B[100, 200]

C30

D[10, 20]

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the address and values arguments.

Using incorrect address or wrong list of values.

The first argument is the starting address 20, the second is the list of values to write.

5fill in blank

hard

Fill all three blanks to create a Modbus TCP client, connect, read 3 coils starting at 5, and close the connection.

PLC Programming

client = ModbusTcpClient('[1]')
client.[2]()
result = client.read_coils([3], 3)
client.close()

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

A192.168.0.50

Bconnect

C5

Dstart

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong IP address format.

Using 'start' instead of 'connect' method.

Using string instead of integer for coil address.

The client connects to IP 192.168.0.50, calls connect() method, and reads coils starting at address 5.

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