Batch counting application in PLC Programming - Step-by-Step Execution

Concept Flow - Batch counting application

Start

↓

Check Start Signal

Yes↓

Increment Batch Count

↓

Check if Batch Complete

No↓

Wait for Next Item

↓

Loop Back to Check Start Signal

Yes↓

Reset Batch Count

↓

End or Restart

The program waits for a start signal, increments the batch count for each item, checks if the batch is complete, and resets or continues accordingly.

Execution Sample

PLC Programming

IF StartSignal AND NOT BatchComplete THEN
  BatchCount := BatchCount + 1;
  IF BatchCount >= BatchSize THEN
    BatchComplete := TRUE;
  END_IF;
END_IF;

This code increments the batch count when the start signal is active and sets batch complete when the count reaches the batch size.

Execution Table

Step	StartSignal	BatchCount Before	Condition (StartSignal?)	Action	BatchCount After	BatchComplete
1	FALSE	0	FALSE	No increment	0	FALSE
2	TRUE	0	TRUE	Increment BatchCount	1	FALSE
3	TRUE	1	TRUE	Increment BatchCount	2	FALSE
4	TRUE	2	TRUE	Increment BatchCount	3	FALSE
5	TRUE	3	TRUE	Increment BatchCount	4	FALSE
6	TRUE	4	TRUE	Increment BatchCount	5	TRUE
7	TRUE	5	TRUE	BatchComplete TRUE, no increment	5	TRUE
8	FALSE	5	FALSE	No increment	5	TRUE

💡 BatchCount reached BatchSize 5 at step 6, BatchComplete set to TRUE, counting stops.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	After 8
BatchCount	0	0	1	2	3	4	5	5	5
BatchComplete	FALSE	FALSE	FALSE	FALSE	FALSE	FALSE	TRUE	TRUE	TRUE
StartSignal	FALSE	FALSE	TRUE	TRUE	TRUE	TRUE	TRUE	TRUE	FALSE

Key Moments - 3 Insights

Why does BatchCount not increase after reaching BatchSize?

As shown in execution_table step 7, once BatchComplete is TRUE, the program stops incrementing BatchCount to avoid counting beyond the batch size.

What happens if StartSignal is FALSE?

At steps 1 and 8 in execution_table, when StartSignal is FALSE, no increment happens and BatchCount remains unchanged.

How does the program know when a batch is complete?

When BatchCount reaches or exceeds BatchSize (step 6), BatchComplete is set to TRUE, signaling batch completion.

Visual Quiz - 3 Questions

Look at the execution_table at step 4. What is the BatchCount after this step?

A 4

B 2

C 3

D 1

💡 Hint

Check the 'BatchCount After' column for step 4 in the execution_table.

At which step does BatchComplete become TRUE?

A Step 5

B Step 6

C Step 7

D Step 8

💡 Hint

Look at the 'BatchComplete' column in execution_table and find when it changes to TRUE.

If StartSignal is FALSE at step 3, what would happen to BatchCount?

A BatchCount stays the same

B BatchCount increments by 1

C BatchCount resets to 0

D BatchComplete becomes TRUE

💡 Hint

Refer to steps where StartSignal is FALSE in execution_table and observe BatchCount changes.

Concept Snapshot

Batch counting application in PLC:
- Wait for StartSignal to be TRUE
- Increment BatchCount each time StartSignal is TRUE
- When BatchCount >= BatchSize, set BatchComplete TRUE
- Stop counting after batch complete
- Reset BatchCount to start new batch

Full Transcript

This batch counting application waits for a start signal to begin counting items in a batch. Each time the start signal is active, the batch count increases by one. When the batch count reaches the predefined batch size, the program marks the batch as complete and stops counting further. If the start signal is not active, the count remains unchanged. This logic ensures accurate batch tracking in automation processes.

Practice

1. In a batch counting application, what is the main purpose of the batch counter variable?

easy

A. To control the speed of the conveyor belt

B. To store the maximum batch size allowed

C. To keep track of how many items have been processed in the current batch

D. To reset the machine automatically

Solution

1. Step 1: Understand the role of the batch counter

   The batch counter variable is used to count items processed in the current batch.

2. Step 2: Differentiate from other variables

   Other options describe different functions, but the counter specifically tracks processed items.

3. Final Answer:

   To keep track of how many items have been processed in the current batch -> Option C

4. Quick Check:

   Batch counter = processed items count [OK]

Hint: Batch counter always counts processed items, not settings or controls [OK]

Common Mistakes:

• Confusing batch counter with batch size limit
• Thinking it resets the machine
• Assuming it controls hardware speed

2. Which of the following is the correct way to increase a batch count variable named BatchCount by 1 in a PLC structured text program?

easy

A. BatchCount := BatchCount + 1;

B. BatchCount += 1;

C. BatchCount++

D. BatchCount =+ 1;

Solution

1. Step 1: Identify correct syntax for increment in PLC structured text

   PLC structured text uses assignment with addition: variable := variable + 1;

2. Step 2: Check each option

   BatchCount := BatchCount + 1; uses correct syntax; the += and ++ options are common in other languages but not standard PLC ST; =+ is a syntax error.

3. Final Answer:

   BatchCount := BatchCount + 1; -> Option A

4. Quick Check:

   Increment in PLC ST = variable := variable + 1 [OK]

Hint: Use full assignment syntax: var := var + 1 in PLC ST [OK]

Common Mistakes:

• Using ++ which is not valid in PLC ST
• Using += which is not standard in PLC ST
• Typing =+ instead of := or =

3. Given the following PLC code snippet for batch counting:

IF ItemDetected THEN
  BatchCount := BatchCount + 1;
END_IF;
IF BatchCount >= BatchSize THEN
  BatchComplete := TRUE;
END_IF;

What will be the value of BatchComplete after processing 5 items if BatchSize is 5 and BatchComplete starts as FALSE?

medium

A. FALSE

B. TRUE

C. Undefined

D. Error

Solution

1. Step 1: Count increments for each detected item

   Each item detected increases BatchCount by 1. After 5 items, BatchCount = 5.

2. Step 2: Check batch completion condition

   BatchComplete is set TRUE if BatchCount >= BatchSize. Since 5 >= 5, BatchComplete becomes TRUE.

3. Final Answer:

   TRUE -> Option B

4. Quick Check:

   BatchCount 5 >= BatchSize 5 -> BatchComplete TRUE [OK]

Hint: BatchComplete becomes TRUE when count reaches batch size [OK]

Common Mistakes:

• Assuming BatchComplete stays FALSE until reset
• Confusing >= with >
• Ignoring initial FALSE value

4. Identify the error in this batch counting PLC code snippet:

IF ItemDetected THEN
  BatchCount := BatchCount + 1
END_IF;
IF BatchCount = BatchSize THEN
  BatchComplete := TRUE;
END_IF;

medium

A. Missing semicolon after increment statement

B. Using single equals (=) instead of double equals (==) for comparison

C. BatchComplete should be reset to FALSE inside the IF

D. BatchCount should be reset before increment

Solution

1. Step 1: Check syntax for increment statement

   The line 'BatchCount := BatchCount + 1' is missing a semicolon at the end, which is required in PLC structured text.

2. Step 2: Verify comparison operator usage

   In PLC ST, '=' is used for comparison, not '==', so this is correct.

3. Final Answer:

   Missing semicolon after increment statement -> Option A

4. Quick Check:

   Semicolon required after statements in PLC ST [OK]

Hint: Always end PLC ST statements with semicolon [OK]

Common Mistakes:

• Confusing '=' with '==' in PLC ST
• Forgetting semicolons after statements
• Assuming reset needed inside IF without context

5. You want to design a batch counting program that resets BatchCount to zero automatically when BatchComplete is TRUE and a ResetButton is pressed. Which code snippet correctly implements this logic in PLC structured text?

hard

A. IF ResetButton THEN
BatchCount := 0;
BatchComplete := FALSE;
END_IF;

B. IF BatchComplete OR ResetButton THEN
BatchCount := 0;
BatchComplete := FALSE;
END_IF;

C. IF BatchComplete THEN
BatchCount := 0;
END_IF;
IF ResetButton THEN
BatchComplete := FALSE;
END_IF;

D. IF BatchComplete AND ResetButton THEN
BatchCount := 0;
BatchComplete := FALSE;
END_IF;

Solution

1. Step 1: Understand the reset condition

   The reset should happen only when both BatchComplete is TRUE and ResetButton is pressed simultaneously.

2. Step 2: Analyze each option's logic

   IF BatchComplete AND ResetButton THEN BatchCount := 0; BatchComplete := FALSE; END_IF; uses AND to require both conditions; the OR version resets too early; the separated IF version causes partial resets; the ResetButton-only version ignores batch completion.

3. Final Answer:

   IF BatchComplete AND ResetButton THEN BatchCount := 0; BatchComplete := FALSE; END_IF; -> Option D

4. Quick Check:

   Reset only when batch complete AND reset pressed [OK]

Hint: Use AND to combine batch complete and reset button conditions [OK]

Common Mistakes:

• Using OR instead of AND causing premature reset
• Separating conditions causing partial resets
• Resetting without checking batch completion