Cascading timers for long delays in PLC Programming - Deep Dive

Overview - Cascading timers for long delays

What is it?

Cascading timers is a method used in PLC programming to create very long delays by linking multiple timers together. Each timer triggers the next one after its delay finishes, allowing the total delay to be much longer than a single timer can handle. This technique helps overcome the time limit of individual timers in PLCs. It is like passing a baton in a relay race where each runner covers part of the distance.

Why it matters

PLCs often have timers with maximum delay limits, which can be too short for some industrial processes needing long waits. Without cascading timers, programmers would struggle to create delays longer than these limits, making some automation tasks impossible or very complex. Cascading timers solve this by combining short timers into a longer total delay, enabling precise control over long wait times in machines and processes.

Where it fits

Before learning cascading timers, you should understand basic PLC programming concepts, especially how timers work individually. After mastering cascading timers, you can explore advanced timing strategies, event sequencing, and optimizing PLC programs for efficiency and reliability.

Mental Model

Core Idea

Cascading timers chain multiple short timers so their delays add up to create a long total delay.

Think of it like...

Imagine a group of friends passing a message down a line, where each friend waits a fixed time before passing it on. The total wait time is the sum of all individual waits.

┌───────────┐     ┌───────────┐     ┌───────────┐
│ Timer 1   │ --> │ Timer 2   │ --> │ Timer 3   │ --> ...
│ Delay T1  │     │ Delay T2  │     │ Delay T3  │
└───────────┘     └───────────┘     └───────────┘
Total Delay = T1 + T2 + T3 + ...

Build-Up - 7 Steps

1

Foundation

Understanding basic PLC timers

Concept: Learn what a timer is in PLCs and how it creates delays.

A PLC timer counts time when activated and signals when its preset time is reached. For example, a TON (Timer ON Delay) starts counting when its input is true and turns its output on after the preset delay. Timers have maximum preset values, often limited by the PLC hardware or software.

Result

You can create delays up to the timer's maximum preset time.

Understanding how a single timer works is essential before combining multiple timers for longer delays.

2

Foundation

Limits of single timers in PLCs

Concept: Recognize the maximum delay a single timer can handle.

Most PLC timers have a maximum preset time, for example, 32767 milliseconds or a few minutes. If your process needs a delay longer than this, a single timer cannot do it. Trying to set a longer delay will either cause errors or unexpected behavior.

Result

You know the maximum delay limit of your PLC timers.

Knowing timer limits helps you realize why cascading timers are necessary for long delays.

3

Intermediate

Basic cascading timer setup

Concept: Chain two timers so the second starts after the first finishes.

Use the output of the first timer as the input trigger for the second timer. For example, Timer1 starts on a condition, and when Timer1 finishes, its output turns on Timer2. Timer2 then counts its delay. The total delay is the sum of Timer1 and Timer2 delays.

Result

A delay longer than a single timer's maximum is achieved by adding two timers.

Chaining timers extends delay duration by adding their times sequentially.

4

Intermediate

Extending to multiple timers

Concept: Add more timers in sequence to increase total delay further.

Repeat the chaining process: each timer's output triggers the next timer. For example, Timer3 starts when Timer2 finishes. This way, you can create very long delays by summing many timers' delays. Each timer handles a manageable delay, avoiding hardware limits.

Result

You can create delays much longer than any single timer's limit.

Breaking a long delay into smaller parts avoids timer overflow and keeps timing accurate.

5

Intermediate

Programming cascading timers in ladder logic

Concept: Implement cascading timers using ladder logic instructions.

In ladder logic, use TON timers with preset times. Connect the first timer's input to your start condition. Connect each subsequent timer's input to the previous timer's done bit. For example: | Start |----[TON Timer1]----(Done1) | Done1 |----[TON Timer2]----(Done2) | Done2 |----[TON Timer3]----(Done3) The final done bit signals the end of the total delay.

Result

A working ladder logic program with cascading timers.

Ladder logic naturally supports cascading timers by using done bits as triggers.

6

Advanced

Handling timer resets and interruptions

🤔Before reading on: do you think resetting one timer in a cascade resets all timers or only that timer? Commit to your answer.

Concept: Manage resets so all timers in the cascade reset properly on interruptions.

If the start condition turns off before the cascade finishes, all timers should reset to avoid partial delays. Implement a reset rung that resets all timers simultaneously when the start condition is false. This prevents timers from continuing unexpectedly and ensures consistent timing on each cycle.

Result

Reliable cascading timers that reset correctly on interruptions.

Proper reset logic prevents timing errors and unpredictable machine behavior.

7

Expert

Optimizing cascading timers for precision and resource use

🤔Before reading on: do you think more timers always mean more precise delays or can it cause issues? Commit to your answer.

Concept: Balance the number of timers and their preset values to optimize accuracy and PLC resource use.

Using many timers increases total delay but can cause cumulative timing errors due to scan time and timer resolution. Choose timer presets carefully to minimize error accumulation. Also, too many timers consume PLC memory and processing power. Sometimes, using a high-resolution timer or a different timing method (like system clock) is better for very long delays.

Result

Efficient cascading timers with minimal timing drift and resource use.

Understanding timer resolution and PLC scan cycles helps avoid subtle timing inaccuracies in long delays.

Under the Hood

Each timer in a PLC counts time based on the PLC's scan cycle and internal clock. When a timer's input condition is true, it increments its count until reaching the preset value, then sets its done bit. Cascading timers use these done bits as triggers for the next timer, effectively chaining delays. The PLC scan cycle and timer resolution affect timing accuracy, and timers reset when their input conditions go false.

Why designed this way?

PLCs have hardware and software limits on timer preset values to keep timing reliable and resource use low. Cascading timers were designed as a simple, modular way to extend delays beyond these limits without complex hardware changes. This approach uses existing timer instructions and logic, making it easy to implement and maintain.

┌───────────────┐     ┌───────────────┐     ┌───────────────┐
│ Start Signal  │ --> │ Timer 1 (TON) │ --> │ Timer 2 (TON) │ --> ...
│ (Input)      │     │ Delay T1      │     │ Delay T2      │
└──────┬────────┘     └──────┬────────┘     └──────┬────────┘
       │ Done1               │ Done2               │ Done3
       └---------------------┴---------------------┘
Total Delay = T1 + T2 + T3 + ...

Myth Busters - 4 Common Misconceptions

Quick: Does resetting one timer in a cascade automatically reset all timers? Commit to yes or no.

Common Belief: Resetting one timer resets the entire cascade automatically.

Reality: Each timer must be reset explicitly; resetting one timer does not reset others unless programmed to do so.

Why it matters: Failing to reset all timers causes partial delays and unpredictable machine timing.

Quick: Can you create an infinitely long delay by adding more timers without any downside? Commit to yes or no.

Common Belief: You can add as many timers as you want to get any delay length without problems.

Reality: Adding many timers increases complexity, resource use, and timing errors due to scan cycle delays and resolution limits.

Why it matters: Ignoring these limits can cause inaccurate delays and overload the PLC.

Quick: Does the done bit of a timer turn on immediately when the preset time is reached? Commit to yes or no.

Common Belief: The done bit activates exactly at the preset time with perfect precision.

Reality: The done bit activates on the next PLC scan after the preset time, causing small timing delays.

Why it matters: Assuming perfect timing can lead to synchronization errors in precise automation tasks.

Quick: Can you use cascading timers to replace all timing functions in a PLC? Commit to yes or no.

Common Belief: Cascading timers are a universal solution for all timing needs in PLCs.

Reality: Cascading timers are only for long delays; other timing functions like pulse timing or interval timing require different methods.

Why it matters: Misusing cascading timers can complicate programs and reduce efficiency.

Expert Zone

1

Timer resolution and PLC scan time cause cumulative delay errors in long cascades, which experts compensate for by adjusting presets or using system clocks.

2

Some PLCs support high-range timers or system timers that reduce the need for cascading timers, but cascading remains useful for compatibility and modularity.

3

Reset logic must consider race conditions where timers might start or stop unexpectedly during rapid input changes, requiring careful programming.

When NOT to use

Avoid cascading timers when your PLC supports high-range or system timers that can handle long delays natively. Also, for very precise timing, use hardware timers or external timing modules instead. For complex event sequencing, consider state machines or event-driven programming rather than long delay cascades.

Production Patterns

In real-world PLC programs, cascading timers are often combined with state machines to control machine phases with long waits. They are also used in batch processes where long soak or cure times are needed. Experts use modular timer blocks with reset and fault handling to ensure robustness in production environments.

Connections

State Machines

Builds-on

Understanding cascading timers helps implement timed states in state machines, enabling precise control over process phases.

Event-driven Programming

Alternative approach

Knowing cascading timers clarifies when event-driven methods might be better for handling complex timing and asynchronous events.

Project Management Scheduling

Similar pattern

Cascading timers resemble task dependencies in project schedules, where one task starts after another finishes, helping cross-domain understanding of sequential delays.

Common Pitfalls

#1 Not resetting all timers causes partial delays to continue unexpectedly.

Wrong approach:

IF NOT StartCondition THEN
Reset Timer1
END_IF

Correct approach:

IF NOT StartCondition THEN
Reset Timer1
Reset Timer2
Reset Timer3
END_IF

Root cause:

Assuming resetting one timer resets the entire cascade.

#2 Setting timer presets longer than allowed causes errors or unpredictable behavior.

Wrong approach:

Timer1.Preset = 999999999 // Too large for timer limit

Correct approach:

Timer1.Preset = 30000 // Within allowed range, use cascading for longer delays

Root cause:

Ignoring hardware/software timer preset limits.

#3 Triggering next timer directly from start condition instead of previous timer done bit breaks the cascade.

Wrong approach:

Timer2.Input = StartCondition // Incorrect chaining

Correct approach:

Timer2.Input = Timer1.Done // Correct chaining

Root cause:

Misunderstanding how timers trigger each other in cascade.

Key Takeaways

Cascading timers combine multiple short timers to create long delays beyond single timer limits.

Each timer's done bit triggers the next timer, making the total delay the sum of all timers' delays.

Proper reset logic is essential to avoid partial delays and unpredictable timing.

Too many timers can cause timing inaccuracies and consume PLC resources, so balance is key.

Cascading timers are a practical solution in PLC programming but have limits and alternatives for complex timing needs.

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