Cascading timers for long delays in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of cascading timers simulation

Consider a PLC program using two timers: Timer1 (T1) set for 10 seconds and Timer2 (T2) set for 5 seconds. T2 starts only after T1 finishes. What is the total delay before T2's done bit is set?

PLC Programming

T1 preset = 10s; T2 preset = 5s; T2 starts when T1 done;

A 20 seconds

B 5 seconds

C 15 seconds

D 10 seconds

💡 Hint

Think about how the timers run one after another, not at the same time.

Timer1 runs for 10 seconds first. After it finishes, Timer2 runs for 5 seconds. Total delay is 10 + 5 = 15 seconds.

🧠 Conceptual

intermediate

Purpose of cascading timers in PLCs

Why do PLC programmers use cascading timers instead of a single long timer for very long delays?

A To avoid using timers altogether

B To make the program run faster

C To reduce the number of timers used

D To overcome timer preset limits and improve accuracy

💡 Hint

Think about hardware or software limits on timer values.

Many PLC timers have maximum preset values. Cascading timers allow longer delays by chaining shorter timers, improving accuracy and reliability.

🔧 Debug

advanced

Identify the error in cascading timer logic

This PLC timer logic is intended to create a 30-second delay using three 10-second timers in cascade. What is the error? Timer1 starts immediately. Timer2 starts when Timer1 done bit is ON. Timer3 starts when Timer1 done bit is ON. What is wrong?

A No error, logic is correct

B Timer3 should start when Timer2 done bit is ON, not Timer1

C Timer1 should start after Timer3

D Timer2 and Timer3 should start together

💡 Hint

Check the start conditions for each timer carefully.

Timer3 starting on Timer1 done means Timer2 and Timer3 run simultaneously, not in cascade. Timer3 should wait for Timer2 done to ensure sequential timing.

🚀 Application

advanced

Calculate total delay with cascading timers and offsets

You have three timers in cascade: T1 = 8s, T2 = 12s, T3 = 5s. T2 starts 2 seconds after T1 done bit is ON, and T3 starts immediately after T2 done bit is ON. What is the total delay from T1 start to T3 done bit ON?

A 27 seconds

B 25 seconds

C 30 seconds

D 23 seconds

💡 Hint

Add all timer durations plus the 2-second delay before T2 starts.

T1 runs 8s, then 2s delay before T2 starts, T2 runs 12s, then T3 runs 5s. Total = 8 + 2 + 12 + 5 = 27 seconds.

📝 Syntax

expert

Identify the syntax error in timer cascade code snippet

Given this PLC structured text snippet for cascading timers, which option contains the syntax error? 1: IF NOT T1.Q THEN 2: T1(IN:=TRUE, PT:=T#10S); 3: ELSE 4: T2(IN T1.Q, PT:=T#5S); 5: END_IF

A Line 4: Incorrect use of timer input parameter

B Line 5: Missing semicolon

C Line 2: Missing parentheses around timer call

D Line 1: IF statement missing THEN keyword

💡 Hint

Check how timer input parameters are assigned in structured text.

Timer input parameters must be assigned with := inside the timer function call. Using T1.Q directly as IN input without := causes syntax error.

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