Cascading timers for long delays in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the first timer when the input is true.

PLC Programming

IF input_signal THEN [1] := TRUE; END_IF;

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

ATimer1.IN

BTimer1.Q

CTimer1.EN

DTimer1.PT

Attempts:

3 left

💡 Hint

Common Mistakes

Using the timer's output 'Q' to start it.

Trying to set the preset time 'PT' to TRUE.

Using the enable 'EN' which is read-only.

The 'IN' input of a timer is used to start the timer when set to TRUE.

2fill in blank

medium

Complete the code to set the preset time of Timer1 to 10 seconds.

PLC Programming

Timer1.PT := T#[1]s;

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

A100

B1

C1000

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Using 100 or 1000 which represent milliseconds or wrong units.

Using 1 which is too short for 10 seconds.

The preset time is set using time literals like T#10s for 10 seconds.

3fill in blank

hard

Fix the error in the code to cascade Timer2 to start after Timer1 finishes.

PLC Programming

Timer2.IN := Timer1.[1];

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

AIN

BQ

CPT

DET

Attempts:

3 left

💡 Hint

Common Mistakes

Using Timer1.IN which is input, not output.

Using Timer1.PT which is preset time, not a signal.

Using Timer1.ET which is elapsed time, not a trigger.

Timer2 should start when Timer1's output 'Q' is TRUE, indicating Timer1 finished.

4fill in blank

hard

Fill both blanks to create a cascading timer that starts Timer3 after Timer2 finishes with a 5-second preset.

PLC Programming

Timer3.IN := Timer2.[1];
Timer3.PT := T#[2]s;

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

AQ

BIN

C5

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Using Timer2.IN instead of Timer2.Q.

Setting preset time to 10 instead of 5 seconds.

Timer3 starts when Timer2.Q is TRUE, and its preset time is set to 5 seconds.

5fill in blank

hard

Fill all three blanks to define a cascading timer sequence with Timer1 (10s), Timer2 (20s), and Timer3 (30s).

PLC Programming

Timer1.PT := T#[1]s;
Timer2.IN := Timer1.[2];
Timer3.PT := T#[3]s;

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

A10

BQ

C30

D20

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up preset times for timers.

Using Timer1.IN instead of Timer1.Q to start Timer2.

Timer1 preset is 10s, Timer2 starts when Timer1.Q is TRUE, Timer3 preset is 20s.

Practice

1. What is the main purpose of using cascading timers in PLC programming for long delays?

easy

A. To combine multiple short timers to achieve a longer delay

B. To run multiple timers simultaneously for faster operation

C. To reset timers automatically after each cycle

D. To reduce the number of timers used in a program

Solution

1. Step 1: Understand the limitation of single timers

   Single timers have a maximum delay limit, so they cannot handle very long delays alone.

2. Step 2: Explain cascading timers concept

   Cascading timers link several short timers in sequence, where each starts after the previous finishes, to create a longer total delay.

3. Final Answer:

   To combine multiple short timers to achieve a longer delay -> Option A

4. Quick Check:

   Cascading timers extend delay by chaining short timers [OK]

Hint: Think of timers as building blocks for longer delays [OK]

Common Mistakes:

• Assuming timers run at the same time
• Believing timers reset automatically
• Thinking cascading reduces timer count

2. Which of the following is the correct way to start the second timer in a cascading timer setup?

easy

A. Start the second timer when the first timer is running

B. Start the second timer when the first timer's done bit is ON

C. Start the second timer when the first timer's preset value is reached

D. Start the second timer immediately after the first timer starts

Solution

1. Step 1: Identify the trigger for the second timer

   The second timer should start only after the first timer finishes, indicated by its done bit being ON.

2. Step 2: Eliminate incorrect options

   Starting the second timer while the first is running or immediately after it starts is incorrect because it breaks the cascade logic.

3. Final Answer:

   Start the second timer when the first timer's done bit is ON -> Option B

4. Quick Check:

   Second timer triggers on first timer done bit [OK]

Hint: Second timer waits for first timer done bit [OK]

Common Mistakes:

• Starting second timer too early
• Using preset value instead of done bit
• Starting timers simultaneously

3. Given the following cascading timers setup:

Timer1: PT=10s, DN bit set after 10s
Timer2: PT=20s, starts when Timer1.DN is ON
Timer3: PT=30s, starts when Timer2.DN is ON

What is the total delay before Timer3.DN becomes ON?

medium

A. 60 minutes

B. 30 seconds

C. 10 seconds

D. 60 seconds

Solution

1. Step 1: Calculate each timer's delay

   Timer1 delays 10 seconds, Timer2 delays 20 seconds, Timer3 delays 30 seconds.

2. Step 2: Add delays in cascade

   Total delay = 10s + 20s + 30s = 60 seconds.

3. Final Answer:

   60 seconds -> Option D

4. Quick Check:

   Sum of all timer delays = 60 seconds [OK]

Hint: Add all timer delays in sequence for total delay [OK]

Common Mistakes:

• Taking only last timer delay
• Multiplying delays instead of adding
• Confusing seconds with minutes

4. In a cascading timer setup, the second timer never starts even after the first timer finishes. What is the most likely cause?

medium

A. The first timer's done bit is not used as a start condition for the second timer

B. The first timer's preset time is too short

C. The second timer's preset time is longer than the first timer's

D. The second timer's start condition is incorrectly linked to the first timer's running bit

Solution

1. Step 1: Analyze the start condition for the second timer

   The second timer must start when the first timer's done bit is ON; if not, it won't start.

2. Step 2: Identify the error

   If the done bit is not used as the start condition, the second timer never triggers, causing the issue.

3. Final Answer:

   The first timer's done bit is not used as a start condition for the second timer -> Option A

4. Quick Check:

   Second timer needs first timer done bit to start [OK]

Hint: Check if second timer triggers on first timer done bit [OK]

Common Mistakes:

• Confusing running bit with done bit
• Blaming preset times for start issues
• Ignoring start condition linkage

5. You need a delay of 5 minutes in a PLC program, but the maximum timer preset is 1 minute. How can you use cascading timers to achieve this delay?

hard

A. Use a single timer set to 5 minutes, ignoring the max preset limit

B. Use 1 timer set to 1 minute and loop it 5 times without cascading

C. Use 5 timers in sequence, each set to 1 minute, starting the next when the previous timer's done bit is ON

D. Use 2 timers set to 2.5 minutes each, starting simultaneously

Solution

1. Step 1: Understand timer preset limit

   The timer cannot be set above 1 minute, so a single timer cannot create a 5-minute delay.

2. Step 2: Apply cascading timers to extend delay

   Use 5 timers of 1 minute each, starting each next timer when the previous timer's done bit is ON, to sum up to 5 minutes.

3. Final Answer:

   Use 5 timers in sequence, each set to 1 minute, starting the next when the previous timer's done bit is ON -> Option C

4. Quick Check:

   Cascade multiple 1-minute timers to reach 5 minutes [OK]

Hint: Chain timers equal to max preset to reach total delay [OK]

Common Mistakes:

• Trying to set timer above max preset
• Starting timers simultaneously instead of cascading
• Looping single timer without done bit control