PLC-to-PLC communication in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this PLC ladder logic snippet for data transfer?

Given a PLC ladder logic that copies a 16-bit word from PLC1 to PLC2 using a communication register, what will be the value in PLC2's register after execution if PLC1's register holds 0x00FF?

PLC Programming

PLC1_Register = 0x00FF
PLC2_Register = 0x0000
// Communication command executed to copy PLC1_Register to PLC2_Register

A 0x0000

B 0xFF00

C Communication error, no data transferred

D 0x00FF

💡 Hint

Think about how data is copied directly from one register to another without modification.

The communication command copies the exact 16-bit value from PLC1's register to PLC2's register, so PLC2_Register will hold 0x00FF.

🧠 Conceptual

intermediate

Which communication method allows real-time data exchange between two PLCs?

Select the communication method that supports real-time data exchange between two PLCs in an industrial environment.

A Manual USB data copy

B File-based data transfer

C Polling via Ethernet/IP

D Batch data upload

💡 Hint

Real-time means continuous and fast data exchange without manual intervention.

Polling via Ethernet/IP allows PLCs to exchange data continuously and in real-time, unlike file-based or manual methods.

🔧 Debug

advanced

Identify the error in this PLC communication script snippet

This PLC script attempts to send a 32-bit integer from PLC1 to PLC2 but fails. What is the error?

PLC Programming

SEND_DATA := PLC1_Data
COMM_CMD(SEND_DATA, DEST=PLC2)
// PLC2 receives data in a 16-bit register

A Data size mismatch: 32-bit sent but 16-bit received

B Data type conversion error in PLC1_Data

C Missing handshake signal before sending

D Incorrect destination address format

💡 Hint

Check if the data size matches between sender and receiver.

Sending a 32-bit integer to a 16-bit register causes data truncation or failure, which is the error here.

🚀 Application

advanced

How to ensure data integrity in PLC-to-PLC communication?

Which approach best ensures data integrity when two PLCs exchange critical control data?

A Use checksum or CRC verification on transmitted data

B Send data without any verification to speed up communication

C Only send data during system startup

D Use manual data entry to confirm values

💡 Hint

Data integrity means detecting errors during transmission.

Using checksum or CRC helps detect errors in data transmission, ensuring integrity.

💻 Command Output

expert

What is the number of items in the data buffer after this PLC communication sequence?

PLC1 sends 5 data words to PLC2 using a buffer. PLC2 reads 3 words successfully. How many items remain in PLC2's buffer?

PLC Programming

PLC1_Buffer = [10, 20, 30, 40, 50]
PLC2_Buffer = []
// PLC2 receives all 5 words
PLC2 reads 3 words from buffer

A 3

B 2

C 5

D 0

💡 Hint

Subtract the number of read words from total received words.

PLC2 received 5 words and read 3, so 2 words remain in the buffer.

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