Step sequence with timers in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a step sequence in PLC programming?

A step sequence is a method to control a process by moving through a series of steps in order, where each step represents a specific action or state.

beginner

Why are timers used in step sequences?

Timers add delays or control how long a step lasts, allowing the process to wait or perform actions for a set time before moving to the next step.

intermediate

How does a timer typically work in a PLC step sequence?

When a step starts, the timer begins counting. Once the timer reaches the preset time, it signals the sequence to move to the next step.

intermediate

What is the difference between TON and TOF timers in PLCs?

TON (Timer ON Delay) starts timing when enabled and finishes after the preset time. TOF (Timer OFF Delay) starts timing when disabled and finishes after the preset time.

beginner

In a step sequence, what happens if a timer is not used?

Without a timer, the sequence might move too quickly through steps, not allowing enough time for actions to complete or for safe operation.

What does a step sequence control in a PLC?

A The order of process actions

B The color of the PLC screen

C The power supply voltage

D The network connection speed

Step sequences control the order of actions in a process.

What is the main purpose of a timer in a step sequence?

A To delay moving to the next step

B To increase PLC memory

C To change the step order randomly

D To reset the PLC

Timers delay the transition to the next step to allow actions to complete.

Which timer type starts counting when enabled?

A TOF (Timer OFF Delay)

B CTU (Counter Up)

C RTO (Retentive Timer)

D TON (Timer ON Delay)

TON timers start counting when enabled.

What happens if a step sequence moves too fast without timers?

A The sequence reverses automatically

B PLC will shut down

C Actions may not complete properly

D The timer resets

Without timers, steps may change too quickly, causing incomplete actions.

In a step sequence, when does the timer reset?

A When the PLC powers on

B When the step changes

C When the timer reaches zero

D When the user presses a button

Timers reset when the sequence moves to a new step.

Explain how timers help control the flow of a step sequence in a PLC.

Think about why waiting is important in a process.

Describe the difference between TON and TOF timers and when you might use each in a step sequence.

Consider what happens when a timer is turned on or off.

Practice

1. What is the main purpose of using timers in a step sequence in PLC programming?

easy

A. To wait a set time before moving to the next step

B. To immediately jump to the last step

C. To reset the entire program

D. To skip steps randomly

Solution

1. Step 1: Understand the role of timers in step sequences

   Timers are used to create delays between steps, ensuring each step runs for a set time.

2. Step 2: Identify the correct timer function

   The timer waits before allowing the sequence to move to the next step, controlling timing precisely.

3. Final Answer:

   To wait a set time before moving to the next step -> Option A

4. Quick Check:

   Timers delay steps = A [OK]

Hint: Timers pause steps to control sequence timing [OK]

Common Mistakes:

• Thinking timers skip steps
• Assuming timers reset the program
• Believing timers jump to last step

2. Which of the following is the correct way to start a timer in a PLC step sequence?

easy

A. StartTimer(T1, 5s)

B. TimerStart T1 5

C. TON T1, 5s

D. T1 = Timer(5)

Solution

1. Step 1: Recognize standard PLC timer syntax

   In many PLC languages, the TON (Timer ON Delay) instruction is used with a timer name and preset time.

2. Step 2: Compare options to standard syntax

   TON T1, 5s uses 'TON T1, 5s' which matches common PLC timer start syntax.

3. Final Answer:

   TON T1, 5s -> Option C

4. Quick Check:

   TON is standard timer start = C [OK]

Hint: Look for TON keyword to start timers [OK]

Common Mistakes:

• Using incorrect function names
• Missing TON keyword
• Wrong timer parameter format

3. Given this step sequence code snippet:

Step 1: Start TON timer T1 for 3 seconds
Step 2: When T1 done, move to Step 2 and start TON timer T2 for 2 seconds
Step 3: When T2 done, move to Step 3

What is the total time before Step 3 starts?

medium

A. 5 seconds

B. 2 seconds

C. 3 seconds

D. Cannot be determined

Solution

1. Step 1: Add timer durations sequentially

   Step 1 waits 3 seconds, then Step 2 waits 2 seconds before Step 3.

2. Step 2: Calculate total wait time

   Total time = 3 seconds + 2 seconds = 5 seconds.

3. Final Answer:

   5 seconds -> Option A

4. Quick Check:

   3s + 2s = 5s total [OK]

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

Common Mistakes:

• Only counting one timer duration
• Confusing timer start with timer done
• Assuming timers run in parallel

4. You have a step sequence with timers but the sequence moves to the next step immediately without waiting. What is the most likely error?

medium

A. Timer is counting up correctly

B. Timer preset value is set to zero

C. Step sequence is missing

D. PLC power is off

Solution

1. Step 1: Identify why timer causes no delay

   If the timer preset is zero, it finishes instantly, causing no wait.

2. Step 2: Check other options

   Counting up correctly means timer works; missing sequence or power off would stop all operation.

3. Final Answer:

   Timer preset value is set to zero -> Option B

4. Quick Check:

   Zero preset = no delay [OK]

Hint: Check timer preset is not zero for delay [OK]

Common Mistakes:

• Ignoring timer preset value
• Assuming power off causes immediate step
• Confusing timer counting with preset

5. You want a step sequence that:
1. Starts motor for 4 seconds,
2. Waits 3 seconds with motor off,
3. Then starts motor again.

Which timer setup correctly implements this sequence?

hard

A. Step 1: TON T1, 4s; Step 2: TON T2, 3s; Step 3: Motor ON

B. Step 1: Motor ON; Step 2: TON T1, 4s; Step 3: TON T2, 3s; Step 4: Motor ON

C. Step 1: TON T1, 3s; Step 2: Motor ON; Step 3: TON T2, 4s

D. Step 1: Motor ON and TON T1, 4s; Step 2: Motor OFF and TON T2, 3s; Step 3: Motor ON

Solution

1. Step 1: Match motor state with timers per step

   Step 1 must run motor ON while timer T1 counts 4 seconds; Step 2 motor OFF with timer T2 for 3 seconds; Step 3 motor ON again.

2. Step 2: Verify options

   Only Step 1: Motor ON and TON T1, 4s; Step 2: Motor OFF and TON T2, 3s; Step 3: Motor ON correctly pairs motor ON/OFF states with timers in the right steps.

3. Final Answer:

   Step 1: Motor ON and TON T1, 4s; Step 2: Motor OFF and TON T2, 3s; Step 3: Motor ON -> Option D

4. Quick Check:

   Motor state matches timers per step = D [OK]

Hint: Pair motor ON/OFF with timers in each step [OK]

Common Mistakes:

• Not turning motor OFF during wait
• Mixing timer durations
• Skipping motor control in steps