Polling vs report-by-exception in SCADA systems - Visual Side-by-Side Comparison

Process Flow - Polling vs report-by-exception

Start

↓

Polling?

Yes→Send request to device

↓

Device responds with data

↓

Report-by-exception?

Yes→Device sends data only on change

↓

SCADA receives data

↓

End

Polling repeatedly asks devices for data; report-by-exception sends data only when changes happen.

Execution Sample

SCADA systems

1. Polling loop:
   send_request()
   wait_response()
   process_data()
2. Report-by-exception:
   device_detects_change()
   send_update()
   process_data()

Shows how polling requests data regularly, while report-by-exception sends data only on changes.

Process Table

Step	Method	Action	Data Sent	SCADA Receives	Next Step
1	Polling	Send request to device	Request	No	Wait for response
2	Polling	Device responds with current data	Data	Yes	Process data
3	Polling	Process data	None	Yes	Send next request
4	Polling	Send next request	Request	No	Repeat steps 2-4
5	Report-by-exception	Device monitors data	None	No	Wait for change
6	Report-by-exception	Data changes detected	Data update	Yes	Process data
7	Report-by-exception	Process data	None	Yes	Wait for next change
8	Report-by-exception	No data change	None	No	Wait continues
9	Polling	Condition: polling continues	N/A	N/A	Loop or stop
10	Report-by-exception	Condition: no change, no send	N/A	N/A	Wait or stop

💡 Polling stops when manual stop or error; report-by-exception waits indefinitely for changes.

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 6	After Step 7	Final
Request Sent	No	Yes	Yes	No	No	Depends on loop
Data Received	No	Yes	Yes	Yes	Yes	Depends on changes
Data Changed	No	No	No	Yes	No	Depends on device
Waiting State	Yes	No	No	Yes	Yes	Depends on method

Key Moments - 3 Insights

Why does polling send requests even if data hasn't changed?

Polling always sends requests to get current data, shown in execution_table rows 1 and 4, regardless of data changes.

How does report-by-exception reduce network traffic?

Report-by-exception sends data only when changes occur, as seen in rows 6 and 8, avoiding unnecessary data transfer.

What happens if no data changes in report-by-exception?

The device waits silently without sending data, shown in row 8, reducing load compared to polling.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does polling send a request to the device?

A Step 2

B Step 6

C Step 1

D Step 5

💡 Hint

Check the 'Action' column for 'Send request to device' in polling rows.

According to variable_tracker, after which step does 'Data Changed' become Yes?

A After Step 2

B After Step 6

C After Step 3

D After Step 7

💡 Hint

Look at the 'Data Changed' row and see when it switches to Yes.

If the device never detects a change, how does report-by-exception behave according to execution_table?

A Sends no data and waits silently

B Sends data regularly like polling

C Sends data only once at start

D Sends error messages

💡 Hint

Refer to row 8 where 'No data change' leads to no data sent.

Concept Snapshot

Polling repeatedly asks devices for data at set intervals.
Report-by-exception sends data only when changes occur.
Polling uses more network traffic; report-by-exception is efficient.
Polling: SCADA initiates communication.
Report-by-exception: device initiates on change.

Full Transcript

Polling and report-by-exception are two ways SCADA systems get data from devices. Polling means SCADA asks devices regularly for data, even if nothing changed. Report-by-exception means devices send data only when something changes. Polling can cause more network traffic because it sends requests and responses all the time. Report-by-exception reduces traffic by sending updates only on changes. In polling, SCADA controls when data is requested. In report-by-exception, the device controls when to send data. This makes report-by-exception more efficient but requires devices to detect changes. Understanding these methods helps optimize SCADA communication.