PLC-to-SCADA communication in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to read a value from the PLC and send it to SCADA.

PLC Programming

READ [1] FROM PLC INTO SCADA_BUFFER;

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

ATIMER

BDATA

CREGISTER

DCOUNTER

Attempts:

3 left

💡 Hint

Common Mistakes

Using TIMER or COUNTER instead of REGISTER causes wrong data to be read.

The PLC register holds the data to be read and sent to SCADA.

2fill in blank

medium

Complete the code to establish communication protocol between PLC and SCADA.

PLC Programming

SET COMMUNICATION_PROTOCOL TO [1];

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

AMODBUS

BSMTP

CFTP

DHTTP

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing HTTP or FTP which are not typical for PLC communication.

MODBUS is a common protocol used for PLC-to-SCADA communication.

3fill in blank

hard

Fix the error in the code to correctly write data from SCADA to PLC.

PLC Programming

WRITE [1] TO PLC_REGISTER;

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

ASCADA_DATA

BPLC_DATA

CSCADA_COMMAND

DPLC_COMMAND

Attempts:

3 left

💡 Hint

Common Mistakes

Using PLC_DATA or PLC_COMMAND causes writing wrong or no data.

SCADA_DATA is the correct source to write into the PLC register.

4fill in blank

hard

Fill both blanks to create a filter that sends only values above 100 to SCADA.

PLC Programming

IF PLC_VALUE [1] 100 THEN SEND [2] TO SCADA;

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

A>

B<

CPLC_VALUE

DSCADA_BUFFER

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' sends values below 100, which is incorrect.

Sending SCADA_BUFFER instead of PLC_VALUE sends wrong data.

The condition checks if PLC_VALUE is greater than 100, then sends PLC_VALUE to SCADA.

5fill in blank

hard

Fill all three blanks to create a dictionary mapping PLC registers to SCADA tags for values above zero.

PLC Programming

mapping = {{ [1]: [2] for [3] in PLC_REGISTERS if [2] > 0 }}

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

Aregister

Bvalue

Dtag

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'tag' as a key without defining it causes errors.

Mixing variable names causes syntax errors.

We map each register to its value if the value is above zero. 'register' is the key, 'value' is the value, and 'register' is the loop variable.

Practice

1. What is the main purpose of PLC-to-SCADA communication?

easy

A. To program the PLC hardware directly

B. To replace the PLC with a SCADA system

C. To allow remote control and monitoring of machines

D. To connect two PLCs without a SCADA system

Solution

1. Step 1: Understand the role of PLC and SCADA

   PLC controls machines locally, SCADA monitors and controls remotely.

2. Step 2: Identify communication purpose

   Communication enables SCADA to read data and send commands to PLC for remote control.

3. Final Answer:

   To allow remote control and monitoring of machines -> Option C

4. Quick Check:

   PLC-to-SCADA communication = remote control and monitoring [OK]

Hint: Think: Who controls and who monitors remotely? [OK]

Common Mistakes:

• Confusing PLC programming with communication
• Thinking SCADA replaces PLC
• Assuming PLCs communicate without SCADA

2. Which of the following is a common protocol used for PLC-to-SCADA communication?

easy

A. Modbus

B. FTP

C. HTTP

D. SMTP

Solution

1. Step 1: Recall common industrial communication protocols

   Modbus and OPC UA are widely used for PLC-to-SCADA data exchange.

2. Step 2: Identify correct protocol from options

   Modbus is a standard protocol for PLC communication; HTTP, FTP, SMTP are not typical for this use.

3. Final Answer:

   Modbus -> Option A

4. Quick Check:

   Industrial protocol for PLC-SCADA = Modbus [OK]

Hint: Industrial control uses Modbus, not web or email protocols [OK]

Common Mistakes:

• Choosing HTTP thinking it's for all network communication
• Confusing FTP or SMTP with control protocols
• Not knowing Modbus is designed for industrial use

3. Given this PLC script snippet reading a SCADA command:

IF SCADA_Command = 1 THEN
  Motor_Start := TRUE;
ELSE
  Motor_Start := FALSE;
END_IF;

What will be the value of Motor_Start if SCADA_Command is 0?

medium

A. Error

B. TRUE

C. 1

D. FALSE

Solution

1. Step 1: Analyze the IF condition

   The condition checks if SCADA_Command equals 1; if true, Motor_Start is TRUE.

2. Step 2: Evaluate with SCADA_Command = 0

   Since 0 is not equal to 1, ELSE branch runs, setting Motor_Start to FALSE.

3. Final Answer:

   FALSE -> Option D

4. Quick Check:

   SCADA_Command=0 means Motor_Start=FALSE [OK]

Hint: IF equals 1? No? Then FALSE [OK]

Common Mistakes:

• Assuming Motor_Start stays TRUE by default
• Confusing numeric 0 with TRUE
• Expecting error due to zero value

4. Identify the error in this PLC-to-SCADA communication snippet:

IF SCADA_Status = TRUE THEN
  Alarm := TRUE
ELSE
  Alarm := FALSE;
END_IF;

medium

A. Missing semicolon after Alarm := TRUE

B. Incorrect use of ELSE statement

C. SCADA_Status should be compared to 1, not TRUE

D. END_IF is not needed

Solution

1. Step 1: Check statement syntax

   Each assignment must end with a semicolon in PLC languages.

2. Step 2: Locate missing semicolon

   Alarm := TRUE line lacks a semicolon at the end, causing syntax error.

3. Final Answer:

   Missing semicolon after Alarm := TRUE -> Option A

4. Quick Check:

   Semicolon missing after assignment = syntax error [OK]

Hint: Every assignment needs a semicolon [OK]

Common Mistakes:

• Ignoring missing semicolon errors
• Thinking ELSE syntax is wrong
• Confusing boolean TRUE with numeric 1

5. You want to send a temperature value from PLC to SCADA every 5 seconds using Modbus. Which approach is best?

hard

A. Send temperature as a string message over HTTP every 5 seconds

B. Write temperature to a Modbus register and update it every 5 seconds

C. Use SCADA to poll the PLC once and never update

D. Send temperature only when it changes, ignoring timing

Solution

1. Step 1: Understand Modbus data exchange

   Modbus uses registers to hold data; PLC writes values, SCADA reads them.

2. Step 2: Choose update method for regular timing

   Writing temperature to a register and updating every 5 seconds ensures SCADA gets fresh data regularly.

3. Final Answer:

   Write temperature to a Modbus register and update it every 5 seconds -> Option B

4. Quick Check:

   Regular register update = reliable Modbus communication [OK]

Hint: Use Modbus registers for periodic data updates [OK]

Common Mistakes:

• Using HTTP which is not standard for PLC-Modbus
• Polling only once misses updates
• Ignoring timing causes stale data