Cascading timers for long delays in PLC Programming - Step-by-Step Execution

Concept Flow - Cascading timers for long delays

Start

↓

Timer1 starts

↓

Timer1 done?

No→Wait

Yes↓

Timer2 starts

↓

Timer2 done?

No→Wait

Yes↓

Output ON

↓

End

The first timer runs until done, then triggers the second timer. When the second timer finishes, the output activates, creating a long delay by chaining timers.

Execution Sample

PLC Programming

Timer1.PT = T#10s
Timer1.IN = Start
IF Timer1.Q THEN
  Timer2.IN = TRUE
  Timer2.PT = T#20s
END_IF
IF Timer2.Q THEN
  Output = TRUE
END_IF

This code uses two timers in sequence to create a 30-second delay before turning on the output.

Execution Table

Step	Timer1.IN	Timer1.ET	Timer1.Q	Timer2.IN	Timer2.ET	Timer2.Q	Output	Action
1	True	0s	False	False	0s	False	False	Start Timer1
2	True	5s	False	False	0s	False	False	Timer1 running (5s elapsed)
3	True	10s	True	False	0s	False	False	Timer1 done, start Timer2
4	True	10s	True	True	10s	False	False	Timer2 running (10s elapsed)
5	True	10s	True	True	20s	True	False	Timer2 done, output ON
6	True	10s	True	True	20s	True	True	Output is ON
7	False	0s	False	False	0s	False	False	Reset all

💡 Execution stops when both timers are done and output is ON, or when reset occurs.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final
Timer1.IN	False	True	True	True	True	True	False
Timer1.ET	0s	0s	5s	10s	10s	10s	0s
Timer1.Q	False	False	False	True	True	True	False
Timer2.IN	False	False	False	False	True	True	False
Timer2.ET	0s	0s	0s	0s	10s	20s	0s
Timer2.Q	False	False	False	False	False	True	False
Output	False	False	False	False	False	False	False

Key Moments - 3 Insights

Why does Timer2 only start after Timer1 is done?

Because Timer2.IN is set to True only when Timer1.Q becomes True, as shown in execution_table step 3.

What happens if Timer1.IN is turned off before it finishes?

Timer1 resets (Timer1.Q becomes False), so Timer2 never starts, as seen in step 7 where all reset.

Why is the output False until both timers finish?

Output is set True only when Timer2.Q is True, which happens after Timer1 and Timer2 complete, as in step 6.

Visual Quiz - 3 Questions

Look at the execution_table at step 3. What is the value of Timer1.Q?

A Undefined

B True

C False

D Timer1.Q does not exist

💡 Hint

Check the Timer1.Q column in row for step 3.

At which step does the output turn True?

A Step 4

B Step 5

C Step 6

D Step 7

💡 Hint

Look at the Output column in the execution_table rows.

If Timer1.IN stayed False, what would happen to Timer2.IN?

A Timer2.IN would stay False

B Timer2.IN would become True anyway

C Timer2.IN would toggle on and off

D Timer2.IN would cause an error

💡 Hint

Refer to variable_tracker for Timer2.IN values when Timer1.IN is False.

Concept Snapshot

Cascading timers chain delays by starting the next timer only when the previous finishes.
Set Timer2.IN = Timer1.Q to cascade.
Output activates after all timers complete.
Useful for delays longer than one timer's max.
Resetting Timer1 stops the chain.

Full Transcript

This visual execution shows how cascading timers create long delays in PLC programming. Timer1 starts first and runs until its preset time is reached. When Timer1 finishes (Timer1.Q becomes True), it triggers Timer2 to start. Timer2 then runs its preset time. Only after Timer2 finishes does the output turn ON. Variables like Timer1.IN, Timer1.ET, Timer1.Q, Timer2.IN, Timer2.ET, Timer2.Q, and Output change step-by-step, showing the chain reaction. If Timer1 is reset before finishing, Timer2 never starts and output stays OFF. This method allows combining timers to achieve delays longer than a single timer can provide.

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