PLC-to-PLC communication in PLC Programming - Mini Project: Build & Apply

PLC-to-PLC Communication Setup

📖 Scenario: You are working in a factory where two PLCs (Programmable Logic Controllers) need to share data to coordinate their tasks. You will create a simple program to send and receive data between these two PLCs using basic communication tags.

🎯 Goal: Build a PLC program that sets up communication variables, configures a communication status flag, writes data to a send tag, reads data from a receive tag, and finally outputs the received data.

📋 What You'll Learn

Create communication data tags with exact names and values

Add a communication status flag variable

Write data to the send tag using a simple assignment

Read data from the receive tag using a simple assignment

Print or display the received data value

💡 Why This Matters

🌍 Real World

PLC-to-PLC communication is essential in factories where multiple controllers coordinate tasks like assembly lines, packaging, and safety monitoring.

💼 Career

Understanding basic PLC communication helps automation technicians and engineers design reliable control systems that keep industrial processes running smoothly.

1

Create communication data tags

Create two integer variables called send_data and receive_data. Initialize send_data with the value 100 and receive_data with the value 0.

PLC Programming

# Create two integer variables send_data and receive_data with values 100 and 0
# Your code here

Hint

Use simple variable assignments to create the tags.

2

Add communication status flag

Create a boolean variable called comm_status and set it to False to represent the communication status.

PLC Programming

send_data = 100
receive_data = 0
# Create a boolean variable comm_status and set it to False
# Your code here

Hint

This flag will help track if communication is active.

3

Write and read communication data

Assign the value send_data to a variable called plc_send_tag to simulate sending data. Then assign the value plc_receive_tag to receive_data to simulate receiving data. Assume plc_receive_tag has the value 200.

PLC Programming

send_data = 100
receive_data = 0
comm_status = False
# Assign send_data to plc_send_tag
# Assign plc_receive_tag (value 200) to receive_data
# Your code here

Hint

Use simple assignments to simulate data transfer.

4

Output the received data

Print the value of receive_data to display the data received from the other PLC.

PLC Programming

send_data = 100
receive_data = 0
comm_status = False
plc_send_tag = send_data
plc_receive_tag = 200
receive_data = plc_receive_tag
# Print the value of receive_data
# Your code here

Hint

Use the print function to show the received data.

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