Polling vs report-by-exception in SCADA systems - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to show how polling works by regularly checking a sensor value.

SCADA systems

while True:
    sensor_value = read_sensor()
    if sensor_value [1] threshold:
        alert_operator()
    sleep(5)

Drag options to blanks, or click blank then click option'

A>

B!=

C<=

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' will only alert if value equals threshold exactly.

Polling checks if the sensor value is greater than a threshold regularly.

2fill in blank

medium

Complete the code to implement report-by-exception by sending data only when it changes.

SCADA systems

previous_value = None
while True:
    current_value = read_sensor()
    if current_value [1] previous_value:
        send_report(current_value)
        previous_value = current_value
    sleep(5)

Drag options to blanks, or click blank then click option'

A<

B!=

C==

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' will send report only if values are the same, which is wrong.

Report-by-exception sends data only when the current value is different from the previous one.

3fill in blank

hard

Fix the error in this polling loop that causes it to miss alerts.

SCADA systems

while True:
    value = read_sensor()
    if value [1] threshold:
        alert_operator()
    sleep(5)

Drag options to blanks, or click blank then click option'

A>=

B>

C<

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' misses the case when value equals threshold.

Using '>=' ensures alerts trigger when value equals or exceeds the threshold, avoiding missed alerts.

4fill in blank

hard

Fill both blanks to create a report-by-exception dictionary with sensor names and values only if value changed.

SCADA systems

reports = {sensor: [1] for sensor, value in sensors.items() if value [2] last_values.get(sensor)}

Drag options to blanks, or click blank then click option'

Avalue

B==

C!=

Dsensor

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' will include unchanged sensors.

The dictionary includes sensor values where the current value differs from the last reported value.

5fill in blank

hard

Fill all three blanks to build a polling loop that reads sensors, updates last values, and alerts if any value exceeds threshold.

SCADA systems

last_values = {}
while True:
    readings = {sensor: read_sensor(sensor) for sensor in sensors}
    for sensor, value in readings.items():
        if value [1] threshold:
            alert_operator(sensor, value)
        last_values[[2]] = [3]
    sleep(5)

Drag options to blanks, or click blank then click option'

A>

Bsensor

Cvalue

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' will alert for values below threshold, which is incorrect.

The loop alerts when value exceeds threshold, then updates last_values with current sensor and value.

Practice

1. What is the main difference between polling and report-by-exception in SCADA systems?

easy

A. Polling sends data only on changes; report-by-exception requests data regularly.

B. Polling and report-by-exception both send data at fixed intervals.

C. Polling requests data regularly; report-by-exception sends data only on changes.

D. Polling uses less network bandwidth than report-by-exception.

Solution

1. Step 1: Understand polling behavior

   Polling means the system asks for data at fixed times, regardless of changes.

2. Step 2: Understand report-by-exception behavior

   Report-by-exception sends data only when there is a change or event to report.

3. Final Answer:

   Polling requests data regularly; report-by-exception sends data only on changes. -> Option C

4. Quick Check:

   Polling = regular requests, Report-by-exception = event-driven [OK]

Hint: Polling = ask often; report-by-exception = send only on change [OK]

Common Mistakes:

• Confusing which method sends data regularly
• Thinking both methods behave the same
• Assuming report-by-exception always uses more bandwidth

2. Which of the following is the correct syntax to configure polling interval in a SCADA system configuration file?

easy

A. polling-interval = 5 seconds

B. polling_interval = '5s' # Poll every 5 seconds

C. pollingInterval: 5sec

D. polling interval = 5s

Solution

1. Step 1: Identify correct syntax format

   Configuration files often use key = 'value' format with quotes for time strings.

2. Step 2: Check each option for syntax correctness

   polling_interval = '5s' # Poll every 5 seconds uses correct key, equals sign, and quotes with valid time unit '5s'. Others have syntax errors or invalid formats.

3. Final Answer:

   polling_interval = '5s' # Poll every 5 seconds -> Option B

4. Quick Check:

   Correct config syntax uses key = 'value' [OK]

Hint: Config keys use underscores and quotes for values [OK]

Common Mistakes:

• Using spaces instead of underscores in keys
• Missing quotes around time values
• Using colons instead of equals sign

3. Given this SCADA polling setup code snippet:

polling_interval = 10  # seconds
last_value = 100
current_value = 100
if current_value != last_value:
    send_report()
else:
    wait(polling_interval)

What will happen when this code runs?

medium

A. The code causes an error due to missing function definitions.

B. send_report() is never called because values are equal.

C. send_report() is called every 10 seconds regardless of value changes.

D. send_report() is called only if current_value changes from last_value.

Solution

1. Step 1: Analyze the condition

   The code compares current_value and last_value; if different, it calls send_report().

2. Step 2: Check values given

   Both values are 100, so condition is false; else branch waits for polling_interval.

3. Final Answer:

   send_report() is called only if current_value changes from last_value. -> Option D

4. Quick Check:

   Report sent only on value change [OK]

Hint: send_report() triggers only on value difference [OK]

Common Mistakes:

• Assuming send_report() runs every time
• Ignoring the if condition logic
• Thinking wait() causes an error

4. You have a SCADA system using report-by-exception, but it never sends updates even when values change. What is the most likely cause?

medium

A. Exception reporting is disabled or misconfigured.

B. The system is polling too frequently.

C. Network bandwidth is too high.

D. Polling interval is set too low.

Solution

1. Step 1: Understand report-by-exception behavior

   It sends data only when exceptions or changes occur and reporting is enabled.

2. Step 2: Identify cause of no updates

   If no updates are sent despite changes, reporting is likely disabled or misconfigured.

3. Final Answer:

   Exception reporting is disabled or misconfigured. -> Option A

4. Quick Check:

   Disabled reporting stops updates [OK]

Hint: Check if exception reporting is enabled first [OK]

Common Mistakes:

• Confusing polling interval with report-by-exception
• Assuming high bandwidth stops updates
• Thinking frequent polling affects exception reporting

5. A SCADA system monitors temperature sensors. To reduce network load but ensure critical alerts are sent immediately, which approach is best?

hard

A. Use report-by-exception for normal data and polling for critical alerts.

B. Use report-by-exception only, ignoring polling.

C. Use polling only with a very short interval.

D. Use polling with a long interval and no exception reporting.

Solution

1. Step 1: Analyze network load and alert needs

   Polling often increases network load; report-by-exception reduces it by sending only changes.

2. Step 2: Combine methods for best result

   Using report-by-exception for normal data saves bandwidth, while polling ensures critical alerts are checked regularly.

3. Final Answer:

   Use report-by-exception for normal data and polling for critical alerts. -> Option A

4. Quick Check:

   Combine polling and exception reporting for efficiency [OK]

Hint: Mix polling and exception reporting for best balance [OK]

Common Mistakes:

• Using polling only causes high network load
• Ignoring critical alert needs
• Relying solely on report-by-exception may miss urgent data