Function block instances in PLC Programming

Introduction

Function block instances let you reuse a block of code with its own memory. This helps you control many devices or tasks separately but with the same logic.

You want to control multiple motors with the same start/stop logic.

You need to monitor several sensors using the same checking rules.

You want to keep track of different timers running at the same time.

You want to organize your program by grouping related code and data.

You want to avoid rewriting the same code for similar tasks.

Syntax

PLC Programming

FUNCTION_BLOCK FB_Name
VAR
  variable1 : TYPE;
  variable2 : TYPE;
END_VAR
(* Your code here *)
END_FUNCTION_BLOCK

// To create an instance:
fb_instance_name : FB_Name;

A function block defines code and variables together.

Each instance has its own copy of variables, so they work independently.

Examples

This example shows a timer function block and two separate timer instances.

PLC Programming

FUNCTION_BLOCK FB_Timer
VAR
  StartTime : TIME;
  Elapsed : TIME;
END_VAR
(* Timer logic here *)
END_FUNCTION_BLOCK

// Create two instances
Timer1 : FB_Timer;
Timer2 : FB_Timer;

Two motor controls use the same logic but keep their own running state.

PLC Programming

FUNCTION_BLOCK FB_MotorControl
VAR
  IsRunning : BOOL;
END_VAR
(* Motor control logic here *)
END_FUNCTION_BLOCK

MotorA : FB_MotorControl;
MotorB : FB_MotorControl;

Sample Program

This program defines a counter function block with an increment method. It creates two instances, increments them separately, and stores results.

PLC Programming

FUNCTION_BLOCK FB_Counter
VAR
  Count : INT := 0;
END_VAR

METHOD Increment : INT
  Count := Count + 1;
  RETURN Count;
END_METHOD
END_FUNCTION_BLOCK

PROGRAM PLC_PRG
VAR
  Counter1 : FB_Counter;
  Counter2 : FB_Counter;
  Result1 : INT;
  Result2 : INT;
END_VAR

// Increment counters
Result1 := Counter1.Increment();
Result2 := Counter2.Increment();
Result2 := Counter2.Increment();

// Output results
END_PROGRAM

Important Notes

Each function block instance keeps its own data separate from others.

Use instances to handle multiple devices or tasks with the same code.

Remember to call methods or update variables on each instance separately.

Summary

Function block instances let you reuse code with separate memory.

They help manage multiple similar tasks easily.

Each instance works independently with its own data.

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