Cascading timers for long delays in PLC Programming - Mini Project: Build & Apply

Cascading timers for long delays

📖 Scenario: You are programming a PLC (Programmable Logic Controller) to control a machine that needs a very long delay before starting a process. Since the PLC timers have a maximum limit, you will use cascading timers to create a longer delay by linking multiple timers.

🎯 Goal: Build a PLC program using cascading timers to achieve a total delay of 120 seconds by chaining two timers: one for 60 seconds and another for 60 seconds. You will configure the timers, link their outputs, and display the final delay completion.

📋 What You'll Learn

Create two timers named T1 and T2 with preset times of 60 seconds each.

Configure T2 to start only after T1 is done.

Use a boolean variable StartProcess to indicate when the total delay is complete.

Print or display the status of StartProcess at the end.

💡 Why This Matters

🌍 Real World

Cascading timers are used in industrial automation when a process requires delays longer than a single timer can handle.

💼 Career

Understanding cascading timers is important for PLC programmers and automation engineers to design reliable control systems with precise timing.

1

Create two timers with 60 seconds preset

Create two timers called T1 and T2. Set their preset times to 60 seconds each using the PT property.

PLC Programming

# Define timers T1 and T2 with preset times
# Your code here

Hint

Use the Timer() function to create timers and set PT to 60000 milliseconds for 60 seconds.

2

Configure T2 to start after T1 is done

Set the IN input of T1 to True to start it immediately. Then set the IN input of T2 to T1.Q so T2 starts only after T1 finishes.

PLC Programming

T1 = Timer()
T1.PT = 60000  # 60 seconds in milliseconds

T2 = Timer()
T2.PT = 60000  # 60 seconds in milliseconds

# Set T1.IN to True and T2.IN to T1.Q
# Your code here

Hint

Set T1.IN = True to start the first timer immediately. Then set T2.IN = T1.Q so the second timer starts when the first timer is done.

3

Run timers and set StartProcess when delay completes

Call the run() method on both T1 and T2 to update their status. Create a boolean variable StartProcess and set it to True when T2.Q is True.

PLC Programming

T1 = Timer()
T1.PT = 60000  # 60 seconds in milliseconds
T1.IN = True

T2 = Timer()
T2.PT = 60000  # 60 seconds in milliseconds
T2.IN = T1.Q

# Run timers and set StartProcess when T2.Q is True
# Your code here

Hint

Call run() on both timers to update them. Use an if statement to set StartProcess = True when T2.Q is True.

4

Display the StartProcess status

Print the value of the boolean variable StartProcess to show if the total delay is complete.

PLC Programming

T1 = Timer()
T1.PT = 60000  # 60 seconds in milliseconds
T1.IN = True

T2 = Timer()
T2.PT = 60000  # 60 seconds in milliseconds
T2.IN = T1.Q

T1.run()
T2.run()

StartProcess = False
if T2.Q:
    StartProcess = True

# Print StartProcess status
# Your code here

Hint

Use print(StartProcess) to display the status.

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