OPC (OLE for Process Control) in SCADA systems - Step-by-Step Execution

Process Flow - OPC (OLE for Process Control)

Client Requests Data

↓

OPC Server Receives Request

↓

Server Reads Data from PLC or Device

↓

Server Sends Data to Client

↓

Client Receives and Uses Data

↓

Repeat or Close Connection

This flow shows how an OPC client asks for data, the OPC server gets it from devices, and sends it back to the client.

Execution Sample

SCADA systems

1. Client connects to OPC Server
2. Client requests 'Temperature' tag
3. Server reads 'Temperature' from PLC
4. Server sends value 72.5 to Client
5. Client displays value

This example traces a client requesting a temperature value from an OPC server connected to a PLC.

Process Table

Step	Action	Component	Data/Value	Result
1	Connect to OPC Server	Client	N/A	Connection Established
2	Request 'Temperature' tag	Client	Tag='Temperature'	Request Sent
3	Receive request	OPC Server	Tag='Temperature'	Request Received
4	Read 'Temperature' from PLC	OPC Server	PLC Tag='Temperature'	Value=72.5
5	Send value to Client	OPC Server	Value=72.5	Value Sent
6	Receive value	Client	Value=72.5	Value Received
7	Display value	Client	Value=72.5	Value Displayed
8	End or Repeat	Client/Server	N/A	Process Ends or Loops

💡 Process ends after value is displayed or repeats for new requests

Status Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	Final
Connection Status	Disconnected	Connected	Connected	Connected	Connected
Requested Tag	None	Temperature	Temperature	Temperature	Temperature
Value from PLC	None	None	72.5	72.5	72.5
Value at Client	None	None	None	72.5	72.5

Key Moments - 3 Insights

Why does the client need to connect before requesting data?

The client must establish a connection first (see Step 1 in execution_table) so the server can receive and respond to requests.

How does the server get the actual data value?

The server reads the value directly from the PLC device (Step 4), not from the client or cache.

What happens if the client requests a tag that does not exist?

The server cannot find the tag and will return an error or no data, stopping the normal flow after Step 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'Value from PLC' after Step 4?

A Temperature

B 72.5

C None

D Disconnected

💡 Hint

Check the 'Value from PLC' column in variable_tracker after Step 4

At which step does the client receive the requested data value?

A Step 6

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Receive value' action in the execution_table

If the client never connects, what happens to the connection status variable?

A It becomes 'Error'

B It changes to 'Connected'

C It stays 'Disconnected'

D It becomes 'None'

💡 Hint

See variable_tracker 'Connection Status' start and after Step 2

Concept Snapshot

OPC (OLE for Process Control) allows clients to request real-time data from industrial devices via an OPC server.
Clients connect to the server, request tags (data points), and receive current values.
The server reads data directly from devices like PLCs and sends it back.
This enables easy, standardized communication between control systems and software.
Always connect before requesting data, and handle errors if tags are missing.

Full Transcript

OPC (OLE for Process Control) is a way for software clients to get data from industrial devices through an OPC server. The client first connects to the server, then asks for a specific data tag like 'Temperature'. The server receives this request, reads the current value from the connected device such as a PLC, and sends the value back to the client. The client then receives and displays this value. This process can repeat as needed. If the client does not connect first, the server cannot receive requests. If the requested tag does not exist, the server returns an error. This flow ensures standardized, real-time data exchange in industrial automation.

Practice

1. What is the primary purpose of OPC (OLE for Process Control) in SCADA systems?

easy

A. To store historical data in a database

B. To enable communication and data sharing between industrial devices and software

C. To design graphical user interfaces for SCADA systems

D. To encrypt data for secure transmission

Solution

1. Step 1: Understand OPC's role in SCADA

   OPC is designed to allow different industrial devices and software to communicate and share data easily.

2. Step 2: Compare options with OPC's function

   Only To enable communication and data sharing between industrial devices and software describes this communication and data sharing role accurately.

3. Final Answer:

   To enable communication and data sharing between industrial devices and software -> Option B

4. Quick Check:

   OPC = Data sharing protocol [OK]

Hint: OPC connects devices and software for data exchange [OK]

Common Mistakes:

• Confusing OPC with database storage
• Thinking OPC designs user interfaces
• Assuming OPC encrypts data

2. Which of the following is the correct syntax to connect an OPC client to an OPC server in a typical OPC client application?

easy

A. opcClient.connect('opc.tcp://server_address:4840')

B. opcClient.start('server_address')

C. opcClient.openConnection('server_address')

D. opcClient.link('opc.tcp://server_address')

Solution

1. Step 1: Identify standard OPC client connection method

   OPC UA clients commonly use a connect method with a server URL like 'opc.tcp://...'.

2. Step 2: Match syntax to common OPC client libraries

   Only opcClient.connect('opc.tcp://server_address:4840') uses the correct method name and URL format for OPC UA connection.

3. Final Answer:

   opcClient.connect('opc.tcp://server_address:4840') -> Option A

4. Quick Check:

   OPC client connects using connect() with opc.tcp URL [OK]

Hint: OPC clients use connect() with opc.tcp URL [OK]

Common Mistakes:

• Using incorrect method names like start or openConnection
• Missing the OPC TCP protocol prefix
• Using link() which is not standard

3. Given the following OPC client code snippet, what will be the output if the tag 'Temperature' has a current value of 75?

value = opcClient.readTag('Temperature')
print(value)

medium

A. Error: Tag not found

B. 'Temperature'

C. 75

D. opcClient.readTag

Solution

1. Step 1: Understand the readTag function

   The readTag method reads the current value of the specified tag from the OPC server.

2. Step 2: Interpret the print output

   Since 'Temperature' tag value is 75, printing value outputs 75.

3. Final Answer:

   75 -> Option C

4. Quick Check:

   Reading tag returns its value [OK]

Hint: readTag returns current tag value directly [OK]

Common Mistakes:

• Printing tag name instead of value
• Confusing method name with output
• Assuming error without reason

4. You wrote this OPC client code to write a value to a tag but it fails:

opcClient.writeTag('Pressure', 120)

What is the most likely cause of failure?

medium

A. The writeTag method requires three arguments

B. The OPC client is not connected to the server

C. The value 120 is not a valid data type

D. The tag 'Pressure' is read-only on the OPC server

Solution

1. Step 1: Check common writeTag failure reasons

   Writing fails often if the tag is read-only or client lacks permission.

2. Step 2: Evaluate other options

   writeTag usually takes two arguments (tag and value), 120 is valid number, and connection failure would cause different error.

3. Final Answer:

   The tag 'Pressure' is read-only on the OPC server -> Option D

4. Quick Check:

   Write fails if tag is read-only [OK]

Hint: Check tag permissions if write fails [OK]

Common Mistakes:

• Assuming wrong method signature
• Blaming data type without checking tag
• Ignoring connection status

5. You want to monitor changes in the 'FlowRate' tag continuously and trigger an alert if it exceeds 100. Which OPC feature should you use to efficiently implement this?

hard

A. Subscribe to the 'FlowRate' tag to receive updates automatically

B. Poll the 'FlowRate' tag value every second manually

C. Write a script to reset the 'FlowRate' tag periodically

D. Use OPC to write a constant value to 'FlowRate'

Solution

1. Step 1: Identify OPC features for monitoring

   OPC supports subscriptions to tags to get automatic updates on value changes.

2. Step 2: Compare polling vs subscription

   Polling is less efficient and slower; subscription triggers alerts immediately on changes.

3. Final Answer:

   Subscribe to the 'FlowRate' tag to receive updates automatically -> Option A

4. Quick Check:

   Subscription = efficient monitoring [OK]

Hint: Use subscription for real-time tag updates [OK]

Common Mistakes:

• Using polling which wastes resources
• Confusing writing with monitoring
• Ignoring OPC subscription feature