Step sequence with timers in PLC Programming - Deep Dive

Overview - Step sequence with timers

What is it?

A step sequence with timers is a way to control machines or processes by moving through a series of steps, each lasting a set time. Timers help decide how long each step runs before moving to the next. This method is common in automation to ensure tasks happen in order and for the right amount of time. It makes machines work smoothly without needing constant human control.

Why it matters

Without step sequences and timers, machines would need constant supervision or manual switching, which is slow and error-prone. This concept automates timing and order, saving time and reducing mistakes. It allows complex processes to run reliably and safely, improving productivity and consistency in factories and other automated systems.

Where it fits

Before learning step sequences with timers, you should understand basic PLC programming concepts like inputs, outputs, and simple timers. After this, you can learn about more complex control methods like state machines, event-driven programming, or PID control for precise process management.

Mental Model

Core Idea

A step sequence with timers moves through ordered steps automatically, using timers to control how long each step lasts before moving on.

Think of it like...

It's like following a cooking recipe where you do one step, wait a set time (like baking for 10 minutes), then move to the next step without checking the clock constantly.

┌─────────────┐     Timer expires     ┌─────────────┐     Timer expires     ┌─────────────┐
│   Step 1    │ ───────────────────▶ │   Step 2    │ ───────────────────▶ │   Step 3    │
└─────────────┘                      └─────────────┘                      └─────────────┘
       │                                  │                                  │
       │                                  │                                  │
    Actions                           Actions                           Actions
       │                                  │                                  │
       └───────────────────────────────────────────────────────────────────┘

Build-Up - 6 Steps

1

Foundation

Understanding basic PLC timers

Concept: Learn what timers are and how they work in PLCs to measure time intervals.

PLCs have built-in timers that count time when activated. For example, an 'On-Delay Timer' starts counting when its input turns on and activates its output after the set time. Timers help delay actions or measure how long something runs.

Result

You can create delays or timed actions in your PLC program using timers.

Knowing how timers work is essential because they are the building blocks for controlling time-based steps in automation.

2

Foundation

Basics of step sequencing

Concept: Understand how to move through steps one by one in a controlled order.

A step sequence uses a variable or bit to represent the current step. The program checks which step is active and runs the actions for that step. When conditions are met, it moves to the next step by changing the active step variable.

Result

You can control a process by moving through steps in order.

Step sequencing organizes complex tasks into simple, manageable parts executed in order.

3

Intermediate

Combining steps with timers

🤔Before reading on: Do you think timers run independently of steps, or are they controlled within each step? Commit to your answer.

Concept: Learn how to start and reset timers within each step to control step duration.

In each step, you start a timer when the step activates. The timer runs while the step is active. When the timer finishes, the program moves to the next step and resets the timer. This ensures each step lasts a set time.

Result

Steps automatically advance after their timer finishes, without manual intervention.

Understanding that timers are tied to steps prevents timing errors and ensures smooth automatic transitions.

4

Intermediate

Implementing step transitions safely

🤔Before reading on: Should the program allow skipping steps if a timer finishes early? Commit to yes or no.

Concept: Learn to control step transitions only after timers complete and conditions are safe.

The program checks if the timer is done and if all safety conditions are met before moving to the next step. This avoids skipping steps or moving too soon, which could cause errors or unsafe states.

Result

Step transitions happen only when timing and safety conditions are satisfied.

Knowing to combine timers with safety checks prevents accidents and process failures.

5

Advanced

Handling step sequence resets and loops

🤔Before reading on: Do you think a step sequence should always restart from step 1 after finishing, or can it stop? Commit to your answer.

Concept: Learn how to reset or loop the sequence based on process needs.

After the last step, the program can reset the sequence to step 1 to repeat or stop the sequence. Resetting involves clearing timers and step variables. Looping is useful for continuous processes; stopping is for batch or one-time tasks.

Result

You can control whether the process repeats or ends after completing all steps.

Understanding reset and loop logic allows flexible control for different automation scenarios.

6

Expert

Optimizing step sequences with timer drift handling

🤔Before reading on: Do you think timers in PLCs are perfectly accurate over long runs? Commit to yes or no.

Concept: Learn how to handle small timing errors (drift) that accumulate over many cycles.

PLC timers can drift slightly due to scan time and processing delays. Over many cycles, this can cause timing to shift. Experts add correction logic or use real-time clocks to keep timing accurate. They may also monitor timer values and adjust steps dynamically.

Result

Step sequences maintain accurate timing even over long or repeated runs.

Knowing timer drift and how to correct it prevents subtle bugs that affect process quality and timing.

Under the Hood

PLCs execute programs in a continuous loop called a scan cycle. During each scan, the PLC reads inputs, executes logic, updates outputs, and processes timers. Timers count elapsed time based on the scan cycle frequency. Step sequences use internal bits or variables to track the current step. When a timer reaches its preset value, the PLC logic changes the step variable to move to the next step. This cycle repeats, controlling the process flow.

Why designed this way?

PLCs were designed for reliable, real-time control in industrial environments. Using scan cycles and timers allows simple, predictable timing without complex hardware. Step sequences with timers provide a clear, modular way to control processes step-by-step, making troubleshooting and maintenance easier. Alternatives like event-driven systems were less common or harder to implement in early PLCs.

┌───────────────┐ Scan Cycle ┌───────────────┐
│ Read Inputs   │──────────▶│ Execute Logic │
└───────────────┘           └───────────────┘
        │                          │
        ▼                          ▼
┌───────────────┐           ┌───────────────┐
│ Update Timers │           │ Update Outputs│
└───────────────┘           └───────────────┘
        │                          │
        └───────────────┬──────────┘
                        ▼
               ┌─────────────────┐
               │ Step Sequence   │
               │ Control Logic   │
               └─────────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Does a timer keep running even if the step changes? Commit to yes or no.

Common Belief: Timers run independently and keep counting even if the step changes.

Reality: Timers are usually reset or stopped when the step changes, so they only count during their assigned step.

Why it matters: If timers run outside their step, steps may advance too early or late, causing process errors.

Quick: Can you use a single timer for multiple steps without resetting? Commit to yes or no.

Common Belief: One timer can be shared across multiple steps without resetting between them.

Reality: Each step should have its own timer or the timer must be reset at each step to avoid incorrect timing.

Why it matters: Sharing timers without resets causes wrong durations and unpredictable step transitions.

Quick: Is it safe to move to the next step immediately when the timer finishes, without checking other conditions? Commit to yes or no.

Common Belief: Once the timer finishes, the program should immediately move to the next step.

Reality: Step transitions should also check safety and process conditions before moving on, not just timer completion.

Why it matters: Ignoring safety checks can cause unsafe machine states or damage.

Quick: Do PLC timers have perfect accuracy over long periods? Commit to yes or no.

Common Belief: PLC timers are perfectly accurate and never drift over time.

Reality: Timers can drift slightly due to scan time and processing delays, affecting long-term timing accuracy.

Why it matters: Ignoring timer drift can cause timing errors in long or repeated processes, reducing quality.

Expert Zone

1

Timer preset values should consider PLC scan time to avoid off-by-one scan errors in timing.

2

Using separate timers per step improves clarity and reduces bugs compared to reusing a single timer.

3

Advanced sequences may use conditional branching, not just linear steps, requiring more complex timer management.

When NOT to use

Step sequences with timers are not ideal for processes requiring precise real-time control or variable timing based on sensor feedback. In such cases, event-driven programming or PID control loops are better alternatives.

Production Patterns

In industry, step sequences with timers are used for batch processes like filling, mixing, or packaging. They often integrate with safety interlocks and alarms. Programmers use structured text or ladder logic with modular timer blocks and state variables for maintainability.

Connections

State Machines

Step sequences with timers are a type of state machine where each step is a state and timers control transitions.

Understanding step sequences helps grasp state machines, which are fundamental in many control and software systems.

Project Management Gantt Charts

Both involve ordered steps with durations and dependencies.

Seeing step sequences like Gantt charts helps understand timing and order in complex projects beyond automation.

Human Workflow Automation

Step sequences with timers automate tasks in machines similar to how humans follow timed steps in workflows.

Recognizing this connection shows how automation principles apply across technology and human processes.

Common Pitfalls

#1 Timer not reset when moving to next step causes incorrect timing.

Wrong approach:

IF Step1Active THEN StartTimer END_IF
IF TimerDone THEN Step1Active = FALSE; Step2Active = TRUE END_IF

Correct approach:

IF Step1Active THEN StartTimer END_IF
IF TimerDone THEN ResetTimer; Step1Active = FALSE; Step2Active = TRUE END_IF

Root cause:

Forgetting to reset the timer means it stays done, causing immediate step changes.

#2 Using one timer for multiple steps without control causes timing overlap.

Wrong approach:

Use Timer1 for Step1 and Step2 without resetting between steps.

Correct approach:

Use Timer1 for Step1 and Timer2 for Step2, or reset Timer1 before Step2 starts.

Root cause:

Sharing timers without resets mixes timing, leading to wrong durations.

#3 Advancing steps immediately on timer done without checking safety.

Wrong approach:

IF TimerDone THEN NextStep END_IF

Correct approach:

IF TimerDone AND SafetyOK THEN NextStep END_IF

Root cause:

Ignoring safety conditions risks unsafe machine operation.

Key Takeaways

Step sequences with timers automate processes by moving through ordered steps controlled by timed delays.

Timers must be started and reset carefully within each step to ensure accurate timing and smooth transitions.

Safety and process conditions should always be checked before advancing steps, not just timer completion.

Understanding PLC scan cycles and timer behavior helps prevent subtle timing errors and drift.

Advanced use includes handling resets, loops, and timer drift correction for reliable long-term automation.

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