Cascading timers for long delays in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of using cascading timers in PLC programming?

Cascading timers are used to create very long delays by linking multiple timers together, each measuring a portion of the total delay time.

beginner

How does a cascading timer system work in simple terms?

When the first timer finishes, it triggers the next timer to start, and this continues until the last timer completes, adding up their times for a long total delay.

intermediate

Why can't a single timer always be used for very long delays in PLCs?

Most PLC timers have a maximum preset time limit, so to achieve longer delays, multiple timers are cascaded to extend the total delay beyond that limit.

beginner

What is a common real-life example where cascading timers might be useful?

In industrial machines, cascading timers can delay a process step for hours or days, like waiting for a chemical reaction to complete before moving to the next step.

intermediate

What should you check when programming cascading timers to avoid errors?

Ensure each timer starts only after the previous one finishes, and verify the total delay matches the needed time by adding all timer presets correctly.

Why do we use cascading timers in PLC programming?

A To create delays longer than a single timer's maximum limit

B To reduce the number of timers used

C To speed up the timer process

D To reset the PLC automatically

Cascading timers allow combining multiple timers to achieve longer delays than one timer can handle alone.

What triggers the second timer in a cascading timer setup?

A When a button is pressed

B When the PLC starts

C When the first timer finishes

D When the last timer finishes

The second timer starts only after the first timer completes its preset delay.

What is a limitation of a single PLC timer that cascading timers help overcome?

A Limited maximum preset time

B Too much power consumption

C Inability to count backwards

D Lack of accuracy

Single timers have a maximum preset time limit; cascading timers extend total delay beyond this limit.

If you need a 10-hour delay but your timer max is 1 hour, how many timers do you need at minimum?

A 20

B 1

C 5

D 10

You need 10 timers each set for 1 hour to reach a total 10-hour delay.

What is important to check when setting up cascading timers?

A Timers are set to zero

B Each timer starts after the previous one finishes

C All timers start at the same time

D Timers are connected to different PLCs

Proper sequencing ensures the total delay is the sum of all timers' delays.

Explain how cascading timers work to create long delays in PLC programming.

Think about how one timer finishing can start another.

Describe a real-world situation where cascading timers would be necessary and why.

Consider processes that take hours or days.

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