PLC-to-PLC communication in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the communication protocol for PLC data exchange.

PLC Programming

COMM_PROTOCOL := [1];

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

ASMTP

BHTTP

CFTP

DEthernet_IP

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing HTTP or FTP which are not typical for PLC communication.

Using SMTP which is for email, not PLC data exchange.

Ethernet/IP is a common protocol used for PLC-to-PLC communication in industrial networks.

2fill in blank

medium

Complete the code to set the PLC communication timeout value in milliseconds.

PLC Programming

PLC_Comm.Timeout := [1];

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

A5000

B10000

C20000

D3000

Attempts:

3 left

💡 Hint

Common Mistakes

Setting timeout too low causing premature disconnections.

Setting timeout too high causing delays in error detection.

A timeout of 5000 milliseconds (5 seconds) is a typical setting for PLC communication to avoid long waits.

3fill in blank

hard

Fix the error in the code to correctly send a data packet from one PLC to another.

PLC Programming

SendDataPacket(PLC1, [1]);

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

ASendBuffer

BDataBuffer()

CDataBuffer

DSendBuffer()

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses which calls the function instead of passing the variable.

Passing the wrong variable name.

The function expects a variable reference, not a function call, so 'DataBuffer' is correct.

4fill in blank

hard

Fill both blanks to configure the PLC communication port and baud rate.

PLC Programming

PLC_Comm.Port := [1];
PLC_Comm.BaudRate := [2];

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

ACOM3

BCOM1

C9600

D115200

Attempts:

3 left

💡 Hint

Common Mistakes

Using a port name that does not exist on the PLC.

Setting an unsupported baud rate.

COM1 is a common default port and 9600 is a standard baud rate for PLC serial communication.

5fill in blank

hard

Fill all three blanks to create a dictionary mapping PLC IDs to their IP addresses for communication setup.

PLC Programming

PLC_IPs := { [1]: [2], [3]: "192.168.1.20" };

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APLC_A

B"192.168.1.10"

CPLC_B

D"192.168.1.15"

Attempts:

3 left

💡 Hint

Common Mistakes

Using IP addresses as keys instead of values.

Mixing up keys and values in the dictionary.

PLC_A and PLC_B are keys, with their respective IP addresses as values in the dictionary.

Practice

1. What is the main purpose of PLC-to-PLC communication?

easy

A. To share data between controllers for coordinated control

B. To program the PLC using a computer

C. To power the PLC devices

D. To store data permanently in the PLC

Solution

1. Step 1: Understand PLC communication basics

   PLC-to-PLC communication allows multiple controllers to exchange data and work together.

2. Step 2: Identify the main goal

   The main goal is to share data for coordinated control, not programming or power supply.

3. Final Answer:

   To share data between controllers for coordinated control -> Option A

4. Quick Check:

   PLC communication = data sharing [OK]

Hint: PLC communication means controllers talk to share data [OK]

Common Mistakes:

• Confusing communication with programming
• Thinking PLC communication powers devices
• Assuming it stores data permanently

2. Which of the following is the correct syntax to define a Modbus TCP client in a PLC script?

easy

A. modbusClient = ModbusTCPClient('192.168.1.10')

B. modbusClient = TCPModbus('192.168.1.10', 502)

C. modbusClient = ModbusTCP('192.168.1.10', 502)

D. modbusClient = Modbus('192.168.1.10', 80)

Solution

1. Step 1: Identify correct class and parameters

   The standard syntax uses ModbusTCP with IP and port 502 for Modbus TCP.

2. Step 2: Check options for correct syntax

   modbusClient = ModbusTCP('192.168.1.10', 502) matches the correct class name and port number for Modbus TCP client.

3. Final Answer:

   modbusClient = ModbusTCP('192.168.1.10', 502) -> Option C

4. Quick Check:

   ModbusTCP + port 502 = correct client syntax [OK]

Hint: Modbus TCP uses port 502 and class ModbusTCP [OK]

Common Mistakes:

• Using wrong class names like TCPModbus
• Using incorrect port numbers
• Omitting the port parameter

3. Given the following PLC script snippet for sending a signal:

modbusClient.write_coil(1, True)
status = modbusClient.read_coil(1)

What will be the value of status after execution?

medium

A. False

B. Error

C. 0

D. True

Solution

1. Step 1: Understand write_coil and read_coil functions

   write_coil(1, True) sets coil 1 to True; read_coil(1) reads its current value.

2. Step 2: Trace the code execution

   After writing True to coil 1, reading coil 1 returns True.

3. Final Answer:

   True -> Option D

4. Quick Check:

   Write True then read coil = True [OK]

Hint: Write True, then read coil returns True [OK]

Common Mistakes:

• Assuming read_coil returns False immediately
• Confusing coil values with integers
• Expecting an error without connection

4. Identify the error in this PLC-to-PLC communication code snippet:

modbusClient = ModbusTCP('192.168.1.20')
modbusClient.write_register(10, 123)

medium

A. Missing port number in ModbusTCP initialization

B. write_register function does not exist

C. IP address format is incorrect

D. Register address 10 is invalid

Solution

1. Step 1: Check ModbusTCP initialization parameters

   ModbusTCP requires IP address and port number (usually 502).

2. Step 2: Identify missing port parameter

   The code only provides IP, missing port number causes connection error.

3. Final Answer:

   Missing port number in ModbusTCP initialization -> Option A

4. Quick Check:

   ModbusTCP needs IP and port [OK]

Hint: Always include port number when creating ModbusTCP client [OK]

Common Mistakes:

• Forgetting the port number argument
• Assuming default port without specifying
• Misreading IP address format

5. You want to synchronize two PLCs so that PLC A sends a temperature value to PLC B every second using Modbus TCP. Which approach best ensures reliable data transfer?

hard

A. PLC A sends temperature as a coil value; PLC B reads it once at startup

B. PLC A writes the temperature to a holding register; PLC B reads it every second with error checking

C. PLC B writes a command to PLC A; PLC A sends temperature only on command

D. PLC A broadcasts temperature on UDP; PLC B listens without confirmation

Solution

1. Step 1: Identify reliable data transfer method

   Using holding registers with regular writes and reads plus error checking is reliable.

2. Step 2: Evaluate other options

   Coils are for binary signals, not values; reading once or UDP without confirmation is unreliable.

3. Final Answer:

   PLC A writes the temperature to a holding register; PLC B reads it every second with error checking -> Option B

4. Quick Check:

   Regular register write/read with error check = reliable [OK]

Hint: Use holding registers and error checks for reliable data [OK]

Common Mistakes:

• Using coils for analog values
• Reading data only once instead of periodically
• Using UDP without confirmation for critical data