EtherNet/IP basics in PLC Programming - Step-by-Step Execution

Concept Flow - EtherNet/IP basics

Start PLC

↓

Initialize Network

↓

Establish TCP Connection

↓

Send/Receive CIP Messages

↓

Process Data

↓

Close Connection

↓

Stop PLC

EtherNet/IP communication starts with PLC initialization, establishing a TCP connection, exchanging CIP messages, processing data, and then closing the connection.

Execution Sample

PLC Programming

1. Open TCP socket to device
2. Send CIP forward open message
3. Wait for forward open response
4. Exchange data messages
5. Close connection

This sequence shows the basic steps a PLC takes to communicate over EtherNet/IP.

Execution Table

Step	Action	Message Sent/Received	Connection State	Data Processed
1	Open TCP socket	TCP SYN sent	Connecting	None
2	Send CIP forward open	Forward Open Request	Connecting	None
3	Receive CIP response	Forward Open Response	Connected	Connection Established
4	Exchange data	CIP I/O Messages	Connected	Data Read/Written
5	Close connection	TCP FIN sent	Disconnected	None

💡 Connection closed after data exchange completes

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
Connection State	None	Connecting	Connecting	Connected	Connected	Disconnected
Data Processed	None	None	None	Connection Established	Data Read/Written	None

Key Moments - 3 Insights

Why do we need to send a 'Forward Open' message after opening the TCP socket?

Opening the TCP socket only establishes a network connection. The 'Forward Open' message sets up the EtherNet/IP session and communication path, as shown in steps 2 and 3 of the execution_table.

What happens if the 'Forward Open Response' is not received?

Without the response (step 3), the connection is not fully established, so data exchange (step 4) cannot proceed. The PLC must handle this by retrying or closing the connection.

Why is the connection state 'Connected' during data exchange?

Because the TCP connection and EtherNet/IP session are active, allowing CIP messages to be sent and received, as shown in step 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the connection state after sending the Forward Open message?

A Connecting

B Connected

C Disconnected

D None

💡 Hint

Check the 'Connection State' column at step 2 in the execution_table.

At which step does the PLC start processing data?

A Step 1

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Data Processed' column in the execution_table.

If the TCP socket fails to open, how would the connection state change in variable_tracker?

A It would become 'Disconnected' after Step 1

B It would remain 'None' after Step 1

C It would become 'Connected' after Step 1

D It would become 'Connecting' after Step 5

💡 Hint

Refer to the 'Connection State' row in variable_tracker and consider what happens if Step 1 fails.

Concept Snapshot

EtherNet/IP basics:
- Start by opening a TCP socket
- Send a 'Forward Open' CIP message to establish session
- Exchange CIP I/O messages for data
- Close connection when done
- Connection state changes: None -> Connecting -> Connected -> Disconnected

Full Transcript

EtherNet/IP communication in PLC programming begins with starting the PLC and initializing the network. The PLC opens a TCP socket to the target device, which moves the connection state to 'Connecting'. Next, it sends a 'Forward Open' CIP message to establish the EtherNet/IP session, changing the state to 'Connected' once the response is received. After the session is established, the PLC exchanges CIP I/O messages to read and write data. Finally, the PLC closes the TCP connection, ending the communication and setting the state to 'Disconnected'. This process ensures reliable data exchange over the network using standard TCP/IP protocols combined with CIP messaging.

Practice

1. What is the main purpose of EtherNet/IP in industrial automation?

easy

A. To provide wireless communication between devices

B. To allow machines to share data over Ethernet networks

C. To replace all PLC programming languages

D. To encrypt data for internet security

Solution

1. Step 1: Understand EtherNet/IP function

   EtherNet/IP is designed to connect industrial devices over Ethernet to share data.

2. Step 2: Compare options

   Only To allow machines to share data over Ethernet networks correctly describes this purpose; others describe unrelated functions.

3. Final Answer:

   To allow machines to share data over Ethernet networks -> Option B

4. Quick Check:

   EtherNet/IP = Data sharing over Ethernet [OK]

Hint: EtherNet/IP = Ethernet + Industrial Protocol for data sharing [OK]

Common Mistakes:

• Confusing EtherNet/IP with wireless protocols
• Thinking EtherNet/IP replaces programming languages
• Assuming it is for internet security encryption

2. Which of the following is the correct way to specify an IP address in EtherNet/IP communication?

easy

A. 192.168.1.1000

B. 192.168.1.256

C. 192.168.1

D. 192.168.1.100

Solution

1. Step 1: Recall valid IP address format

   An IP address has four numbers (octets) between 0 and 255 separated by dots.

2. Step 2: Check each option

   192.168.1.100 has valid numbers (192,168,1,100). Options B and D have numbers >255, and C has only three parts.

3. Final Answer:

   192.168.1.100 -> Option D

4. Quick Check:

   Valid IP = four numbers 0-255 [OK]

Hint: IP octets must be 0-255, four parts separated by dots [OK]

Common Mistakes:

• Using numbers greater than 255 in IP address
• Omitting one or more octets
• Using invalid characters or formats

3. Given the following PLC code snippet for reading a tag over EtherNet/IP:

ReadTag("192.168.0.10", "MotorSpeed")

What is the expected result if the tag MotorSpeed has a value of 1500?

medium

A. The function returns 1500

B. The function returns an error code

C. The function returns null

D. The function returns the IP address

Solution

1. Step 1: Understand ReadTag function

   ReadTag reads the value of a tag from a device at the given IP address.

2. Step 2: Check tag value

   The tag MotorSpeed has value 1500, so the function should return this value.

3. Final Answer:

   The function returns 1500 -> Option A

4. Quick Check:

   ReadTag returns tag value = 1500 [OK]

Hint: ReadTag returns the current tag value from device [OK]

Common Mistakes:

• Assuming it returns IP or error without reason
• Confusing tag name with value
• Expecting null if tag exists

4. You wrote this code to write a value to a tag over EtherNet/IP:

WriteTag("192.168.1.5", "PumpStatus", 1)

But the write fails. Which is the most likely cause?

medium

A. The tag name is misspelled or does not exist

B. The value 1 is not allowed for PumpStatus

C. The IP address is invalid

D. EtherNet/IP does not support writing values

Solution

1. Step 1: Check IP address validity

   192.168.1.5 is a valid IP address format, so unlikely the cause.

2. Step 2: Consider tag name correctness

   If the tag name is misspelled or missing, the write will fail.

3. Step 3: Evaluate value and protocol support

   Value 1 is usually valid for status tags; EtherNet/IP supports writing values.

4. Final Answer:

   The tag name is misspelled or does not exist -> Option A

5. Quick Check:

   Incorrect tag name causes write failure [OK]

Hint: Check tag spelling first if write fails [OK]

Common Mistakes:

• Assuming IP address is always wrong
• Thinking EtherNet/IP can't write data
• Ignoring tag name correctness

5. You want to monitor temperature from multiple devices using EtherNet/IP. Which approach best ensures efficient data collection?

hard

A. Poll each device individually in a loop with delays

B. Use a multicast message to request all devices at once

C. Configure a scanner to cyclically read tags from all devices

D. Manually connect to each device and read tags one by one

Solution

1. Step 1: Understand data collection methods

   Polling each device with delays or manual reads is slow and inefficient.

2. Step 2: Evaluate multicast and scanner options

   Multicast is not commonly used for EtherNet/IP tag reads; scanners efficiently cycle through devices automatically.

3. Final Answer:

   Configure a scanner to cyclically read tags from all devices -> Option C

4. Quick Check:

   Scanner cyclic reads = efficient multi-device monitoring [OK]

Hint: Use scanners for automatic cyclic data reads [OK]

Common Mistakes:

• Using manual or delayed polling causing slow updates
• Assuming multicast works for tag reads
• Not using built-in scanner features