Function block instances in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a function block instance named 'Motor1'.

PLC Programming

VAR
  Motor1 : [1];
END_VAR

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

AMotor

BPROGRAM

CCONFIGURATION

DVAR

Attempts:

3 left

💡 Hint

Common Mistakes

Using PROGRAM instead of the function block type.

Using VAR keyword incorrectly.

In PLC programming, a function block instance is declared by specifying the function block type (Motor) after the colon in the VAR section.

2fill in blank

medium

Complete the code to call the 'Start' method of the function block instance 'Motor1'.

PLC Programming

Motor1.[1]();

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

AReset

BStop

CStart

DInit

Attempts:

3 left

💡 Hint

Common Mistakes

Calling 'Stop' instead of 'Start'.

Using 'Init' which might initialize but not start.

To start the motor, you call the 'Start' method on the Motor1 instance.

3fill in blank

hard

Fix the error in the code to correctly assign a value to the 'Speed' property of 'Motor1'.

PLC Programming

Motor1.[1] := 1500;

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

ASpeed

Bspeed

CSetSpeed

DspeedValue

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase 'speed' instead of 'Speed'.

Trying to assign to a method name like 'SetSpeed'.

Property names are case sensitive; 'Speed' with capital S is correct.

4fill in blank

hard

Fill both blanks to declare two instances 'Pump1' and 'Pump2' of the function block 'PumpControl'.

PLC Programming

VAR
  [1] : PumpControl;
  [2] : PumpControl;
END_VAR

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

APump1

BPumpControl

CPump2

DControlPump

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping instance names with type names.

Using incorrect type name 'ControlPump'.

Instance names are 'Pump1' and 'Pump2', both declared as type 'PumpControl'.

5fill in blank

hard

Fill all three blanks to create a function block instance 'Conveyor1', set its 'Speed' to 100, and call its 'Start' method.

PLC Programming

VAR
  [1] : Conveyor;
END_VAR

[1].[2] := 100;
[1].[3]();

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

AConveyor1

BSpeed

CStart

DConveyor

Attempts:

3 left

💡 Hint

Common Mistakes

Using type name instead of instance name in code lines.

Mixing up property and method names.

Declare instance 'Conveyor1' of type 'Conveyor', set 'Speed' property, then call 'Start' method.

Practice

1. What is the main purpose of using function block instances in PLC programming?

easy

A. To reuse code with separate memory for each instance

B. To write code that runs only once

C. To combine multiple programs into one file

D. To create global variables accessible everywhere

Solution

1. Step 1: Understand function block instances

   Function block instances allow you to reuse the same code but keep data separate for each instance.

2. Step 2: Compare options

   Only To reuse code with separate memory for each instance correctly describes this purpose. Other options describe unrelated concepts.

3. Final Answer:

   To reuse code with separate memory for each instance -> Option A

4. Quick Check:

   Function block instances = separate memory reuse [OK]

Hint: Think: same code, different data for each instance [OK]

Common Mistakes:

• Confusing instances with global variables
• Thinking instances run only once
• Mixing instances with program files

2. Which of the following is the correct syntax to declare two separate instances of a function block named TimerFB in IEC 61131-3 style?

easy

A. Timer1 -> TimerFB; Timer2 -> TimerFB;

B. Timer1 : TimerFB; Timer2 : TimerFB;

C. Timer1 = TimerFB(); Timer2 = TimerFB();

D. Timer1 : TimerFB() Timer2 : TimerFB()

Solution

1. Step 1: Recall IEC 61131-3 instance declaration

   Instances are declared with name : FunctionBlockName; syntax.

2. Step 2: Check options

   Timer1 : TimerFB; Timer2 : TimerFB; uses correct syntax with semicolons. Others use invalid or incorrect syntax.

3. Final Answer:

   Timer1 : TimerFB; Timer2 : TimerFB; -> Option B

4. Quick Check:

   Instance declaration uses colon and semicolon [OK]

Hint: Use colon and semicolon to declare instances [OK]

Common Mistakes:

• Using equal sign instead of colon
• Omitting semicolons
• Adding parentheses after function block name

3. Given the following code snippet in a PLC program:

Timer1 : TimerFB;
Timer2 : TimerFB;

Timer1(IN := TRUE);
Timer2(IN := FALSE);

// After one cycle, what is the state of Timer1.Q and Timer2.Q?

medium

A. Timer1.Q = TRUE, Timer2.Q = FALSE

B. Timer1.Q = FALSE, Timer2.Q = TRUE

C. Timer1.Q = TRUE, Timer2.Q = TRUE

D. Timer1.Q = FALSE, Timer2.Q = FALSE

Solution

1. Step 1: Understand instance independence

   Timer1 and Timer2 are separate instances with their own inputs and outputs.

2. Step 2: Analyze input signals

   Timer1.IN is TRUE, so Timer1.Q will become TRUE after timing. Timer2.IN is FALSE, so Timer2.Q remains FALSE.

3. Final Answer:

   Timer1.Q = TRUE, Timer2.Q = FALSE -> Option A

4. Quick Check:

   Separate instances keep outputs independent [OK]

Hint: Each instance acts independently with its own inputs [OK]

Common Mistakes:

• Assuming outputs share state between instances
• Confusing input signals for both instances
• Ignoring instance separation

4. Identify the error in this function block instance declaration:

Timer1 : TimerFB
Timer2 : TimerFB;

medium

A. Incorrect function block name

B. Extra semicolon after Timer2 declaration

C. Using colon instead of equal sign

D. Missing semicolon after Timer1 declaration

Solution

1. Step 1: Check syntax for instance declarations

   Each instance declaration must end with a semicolon.

2. Step 2: Locate the missing semicolon

   Timer1 declaration lacks a semicolon at the end, causing syntax error.

3. Final Answer:

   Missing semicolon after Timer1 declaration -> Option D

4. Quick Check:

   Each declaration ends with semicolon [OK]

Hint: Always end instance declarations with semicolon [OK]

Common Mistakes:

• Omitting semicolon after each instance
• Confusing colon with equal sign
• Assuming semicolon is optional

5. You want to control three independent motors using the same function block MotorControlFB. Which approach correctly uses function block instances to achieve this?

hard

A. Create three copies of MotorControlFB code with different names

B. Declare one instance MotorControlFB and call it three times with different inputs

C. Declare three instances: Motor1, Motor2, Motor3 of MotorControlFB and call each separately

D. Use global variables inside MotorControlFB to handle all three motors

Solution

1. Step 1: Understand instance independence for multiple devices

   Each motor needs its own instance to keep control data separate.

2. Step 2: Evaluate options

   Declare three instances: Motor1, Motor2, Motor3 of MotorControlFB and call each separately declares three separate instances, allowing independent control. Declare one instance MotorControlFB and call it three times with different inputs reuses one instance incorrectly. Use global variables inside MotorControlFB to handle all three motors uses globals, which mixes data. Create three copies of MotorControlFB code with different names duplicates code, which is inefficient.

3. Final Answer:

   Declare three instances: Motor1, Motor2, Motor3 of MotorControlFB and call each separately -> Option C

4. Quick Check:

   Multiple devices = multiple instances [OK]

Hint: One instance per device keeps data separate [OK]

Common Mistakes:

• Trying to reuse one instance for multiple devices
• Using global variables inside function block
• Copy-pasting code instead of using instances