Sequence control from SCADA in SCADA systems - Step-by-Step Execution

Process Flow - Sequence control from SCADA

Start SCADA Sequence

↓

Check Initial Conditions

↓

Send Command to Device 1

↓

Wait for Device 1 Confirmation

↓

Send Command to Device 2

↓

Wait for Device 2 Confirmation

↓

Continue Sequence or Stop

Yes No↓

Next Step

The SCADA system starts a sequence by checking conditions, sending commands step-by-step to devices, waiting for confirmations, and deciding to continue or stop.

Execution Sample

SCADA systems

1. Start sequence
2. Check if pump is ready
3. Turn on pump
4. Wait for pump ON confirmation
5. Open valve
6. Wait for valve open confirmation

This sequence controls a pump and valve step-by-step, ensuring each step completes before moving on.

Process Table

Step	Action	Condition/Check	Result	Next Step
1	Start sequence	None	Sequence started	Check pump readiness
2	Check pump readiness	Pump status == READY	True	Send command: Turn on pump
3	Send command: Turn on pump	Command sent	Pump turning ON	Wait for pump ON confirmation
4	Wait for pump ON confirmation	Pump status == ON	True	Open valve
5	Send command: Open valve	Command sent	Valve opening	Wait for valve open confirmation
6	Wait for valve open confirmation	Valve status == OPEN	True	Sequence complete
7	Sequence complete	All steps done	Sequence ended successfully	End

💡 Sequence ends after all devices confirm successful operation.

Status Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	Final
pump_status	UNKNOWN	READY	ON	ON	ON
valve_status	UNKNOWN	UNKNOWN	UNKNOWN	OPEN	OPEN
sequence_state	NOT_STARTED	IN_PROGRESS	IN_PROGRESS	IN_PROGRESS	COMPLETED

Key Moments - 2 Insights

Why does the sequence wait after sending a command before continuing?

The sequence waits for confirmation (see steps 4 and 6 in execution_table) to ensure the device completed the action before moving on, preventing errors.

What happens if the pump is not ready at step 2?

If the pump is not ready, the condition at step 2 fails and the sequence would not proceed to turn on the pump, preventing unsafe operations.

Visual Quiz - 3 Questions

Look at the execution_table, what is the pump_status after step 4?

A READY

B ON

C OFF

D UNKNOWN

💡 Hint

Check the 'pump_status' variable in variable_tracker after step 4.

At which step does the sequence confirm the valve is open?

A Step 6

B Step 5

C Step 3

D Step 7

💡 Hint

Look at the 'Wait for valve open confirmation' action in execution_table.

If the pump was not ready at step 2, what would happen to the sequence_state?

A It would become COMPLETED

B It would stay IN_PROGRESS

C It would stop before IN_PROGRESS

D It would remain NOT_STARTED

💡 Hint

Refer to the condition check at step 2 and sequence_state changes in variable_tracker.

Concept Snapshot

Sequence control in SCADA:
- Start sequence and check device readiness
- Send command to device
- Wait for device confirmation
- Proceed step-by-step
- Stop when all steps complete
- Ensures safe, ordered operations

Full Transcript

Sequence control from SCADA means the system runs steps one after another. It starts by checking if devices like pumps or valves are ready. Then it sends commands to turn them on or open them. After each command, it waits for confirmation that the device did the action. This way, the system avoids mistakes by making sure each step finishes before the next starts. The sequence ends when all devices confirm success.

Practice

1. What is the main purpose of sequence control in a SCADA system?

easy

A. To run machine steps automatically in a specific order

B. To manually operate each machine step

C. To monitor network traffic only

D. To store historical data without control

Solution

1. Step 1: Understand sequence control function

   Sequence control automates machine steps to run in order without manual intervention.

2. Step 2: Compare options with definition

   Only To run machine steps automatically in a specific order describes running steps automatically in order, matching sequence control purpose.

3. Final Answer:

   To run machine steps automatically in a specific order -> Option A

4. Quick Check:

   Sequence control = automatic ordered steps [OK]

Hint: Sequence control means automatic step-by-step operation [OK]

Common Mistakes:

• Confusing manual operation with sequence control
• Thinking sequence control only monitors data
• Assuming sequence control stores data without action

2. Which SCADA command is used to pause a sequence until a condition is met?

easy

A. START

B. STOP

C. WAIT

D. RESET

Solution

1. Step 1: Identify command for pausing sequence

   The WAIT command pauses the sequence until a specified condition or time is met.

2. Step 2: Eliminate other commands

   START begins sequences, STOP ends them, RESET clears states; only WAIT pauses.

3. Final Answer:

   WAIT -> Option C

4. Quick Check:

   Pause sequence = WAIT command [OK]

Hint: WAIT means pause until condition or time met [OK]

Common Mistakes:

• Using START to pause instead of begin
• Confusing STOP with pause
• Thinking RESET pauses sequence

3. Given this SCADA sequence snippet:

STEP 1: START motor
STEP 2: WAIT until temperature > 50
STEP 3: STOP motor

What happens if temperature never exceeds 50?

medium

A. Sequence pauses indefinitely at STEP 2

B. Sequence skips STEP 2 and stops motor

C. Motor stops immediately

D. Motor runs continuously without stopping

Solution

1. Step 1: Analyze WAIT condition

   WAIT pauses sequence until temperature > 50 is true.

2. Step 2: Consider temperature never exceeds 50

   If condition never met, sequence stays paused at STEP 2 indefinitely.

3. Final Answer:

   Sequence pauses indefinitely at STEP 2 -> Option A

4. Quick Check:

   WAIT blocks progress until condition true [OK]

Hint: WAIT holds sequence until condition true, else pause [OK]

Common Mistakes:

• Assuming sequence skips WAIT step
• Thinking motor stops immediately
• Believing motor runs nonstop without control

4. Identify the error in this SCADA sequence:

STEP 1: START pump
STEP 2: WAIT until pressure < 30
STEP 3: WAIT until pressure > 40
STEP 4: STOP pump

What is the main problem?

medium

A. Sequence never starts pump

B. WAIT conditions can cause deadlock if pressure stays between 30 and 40

C. STOP pump command is missing

D. Pressure conditions are reversed

Solution

1. Step 1: Review WAIT conditions

   STEP 2 waits for pressure < 30, STEP 3 waits for pressure > 40.

2. Step 2: Consider pressure between 30 and 40

   If pressure stays between 30 and 40, neither WAIT condition is met, causing sequence to pause indefinitely (deadlock).

3. Final Answer:

   WAIT conditions can cause deadlock if pressure stays between 30 and 40 -> Option B

4. Quick Check:

   Conflicting WAITs cause deadlock [OK]

Hint: Conflicting WAITs cause sequence to freeze [OK]

Common Mistakes:

• Ignoring deadlock possibility
• Thinking STOP command is missing
• Assuming pressure conditions are reversed

5. You want to design a SCADA sequence to fill a tank safely:

1. OPEN valve
2. WAIT until level >= 80%
3. CLOSE valve
4. WAIT 10 seconds
5. START mixer

Which improvement ensures safety if the level sensor fails and reads constant 0%?

hard

A. Ignore sensor and rely on manual control

B. Remove WAIT steps to avoid delays

C. Start mixer immediately after opening valve

D. Add a timeout after WAIT to close valve if level not reached

Solution

1. Step 1: Understand sensor failure risk

   If level sensor fails at 0%, WAIT until level >= 80% never completes, valve stays open indefinitely.

2. Step 2: Add timeout to handle failure

   Adding a timeout after WAIT ensures valve closes even if sensor fails, preventing overflow or damage.

3. Final Answer:

   Add a timeout after WAIT to close valve if level not reached -> Option D

4. Quick Check:

   Timeout prevents infinite wait on sensor failure [OK]

Hint: Use timeout to avoid infinite wait on sensor failure [OK]

Common Mistakes:

• Removing WAIT risks unsafe operation
• Starting mixer too early causes errors
• Ignoring sensor failure risks overflow