PLC Timer

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PLC – automated control for the industry

Programmable logic controllers monitor and regulate machines and manufacturing processes in industrial environments with automation technology. In the future, as autonomous systems, they could even optimize processes intelligently and independently. In our guide, we introduce you to the various systems and provide tips on selection and the right accessories.


  • What role does the programmable logic controller play in automation?
  • Advantages of SPS
  • How is a programmable logic controller structured?
  • Functions of programmable logic controllers
  • Compatibility and accessories for networking
  • The right equipment for your PLC
  • types of PLC
  • Modular PLC: A simple and individual solution for every machine system
  • Programming languages ​​for PLC

What role does the programmable logic controller play in automation?

A PLC is a digital electronic system that is equipped with programmable memory. The respective control instruction for the appropriate function can then be saved on this. Various types of machines and processes are then controlled via digital or analogue inputs and outputs.
Thanks to their great flexibility, PLCs are being used more and more frequently in many areas of industry and are replacing conventional connection-programmable controllers. Instead of hard-wired logic based on relay switches, where the control function can only be changed by laborious switching, these electronic components can be digitally programmed and take on complex control tasks in networked production processes of Industry 4.0.
Typical examples of the use of PLCs can be found in production plants, packaging machines, in the beverage and steel industries and also in the medical sector. But you also come across digital electrical controls in everyday life, for example in traffic light circuits, escalators, elevators, roller shutters and gate systems or even in heating systems.

Advantages of PLC

  • high flexibility and versatility
  • uncomplicated duplication and modification of the programming instead of cumbersome rewiring
  • simplified implementation of programming
  • less assembly effort
  • quick change of function
  • high reliability
  • low energy consumption
  • little need for space
  • Networking options with other devices and systems
  • Remote diagnosis and maintenance are possible

Programmable logic controllers only have a few disadvantages, which in individual cases speak for the use of conventional connection-programmable controllers: The costs for the components are comparatively high so the purchase of PLCs is not worthwhile for many small controllers. In addition, an appropriate technical infrastructure with digital devices and trained personnel are required for effective use. Acceptance and commissioning of systems can also be more complex since the program routines and programming have to be tested in addition to the machine components. Individual components such as safety circuits or device monitoring may become more complex.

How is a programmable logic controller structured?

The basic version of a PLC consists of a processor in the central module, signal inputs and outputs, an operating system and the interface.
The control is programmed via the application program on the computer or via a connected control panel. It is loaded onto the PLC via the interface and defines the switching of inputs and outputs. The actual operation then takes place independently of the computer and, thanks to its own PLC power supply, also autonomously.
Depending on the complexity, PLCs have a different number of analogue or digital inputs and outputs, which are linked to the machine or system via so-called sensors and actuators. At the inputs, for example, sensors such as pressure sensors, temperature or level sensors monitor the machine functions. The operating system evaluates the information collected, compares it with the parameters programmed by the user and sends the appropriate control signals to the outputs.
The actuators that control the functions are connected here. These can be contactors for switching on electric motors, electric valves for compressed air and hydraulics or drive control modules.

Functions of programmable logic controllers

PLCs can take on specific functions, such as:

  • link control,
  • Flow control,
  • as well as time, counting and arithmetic functions.

Cycle-oriented PLCs work strictly according to EVA, the basic principle of data processing with input, processing, output. The inputs are queried and control is passed to the user program. After transferring the control signals to the outputs, the process starts over.
Cyclic PLCs with interrupt processing report an alarm when the status of the connected sensor changes and then start an additional program loop adapted to the respective situation before continuing the main program.
Event-controlled PLCs process-specific pre-programmed tasks after a status change of the connected sensors.

Compatibility and accessories for networking

One of the biggest advantages of PLC is the cross-system networking with other devices and computers. Correct networking is not only important for new installations. Even PLCs based on older standards can be connected to modern devices during retrofitting using the appropriate PLC cables, plugs and adapters.
Most PLCs can be connected to a PC via a serial cable. Serial device servers are suitable for this, with which conventional serial components can be connected via LAN. Ethernet media converters connect different transmission media with each other in a compatible way. Common interfaces are RS-232 and RS-485 with DB-9 and DB-25 connectors.
However, PLC interfaces are not standardized, so depending on the device provider, other solutions are used in addition to standard serial cables, which you should find out about in advance or involve a specialist. In general, the design thus determines the requirements for the accessories for the respective PLC

The right equipment for your PLC

Your PLC gains in efficiency with the right additional equipment and can also be expanded with devices via the interface.

  • DIN rails: for optimal assembly.
  • Displays and monitors: for the direct display of operational data.
  • HMI touch panel: for multifunctional visualization, operation and diagnosis of machines and systems.
  • Smart gateway: Networking machines and systems intelligently in the IoT.

types of PLC

In terms of structure and functionality, a distinction is made between hard PLCs and soft PLCs as well as compact and slot PLCs.
Classic PLCs are hardware-based, hence their name Hard-PLC. They usually only include the most important control functions, but extensions for DIN rails and plug connections are quickly possible.
Soft PLCs take over the control in software form. They have their own operating system and often additional user software. However, they do not have their own CPU, but use the PC processor and have to share the processing power with the PC’s operating system and, if necessary, other applications. This can have an impact on the performance of the respective PLC when the load is high.
With a compact PLC, all components are housed together on a single circuit board in a single housing.
A slot PLC is used directly as a PCI plug-in card and for specific tasks such as storing production data. With these controllers, the PC replaces the PLC hardware. In contrast to a soft PLC, however, a slot PLC has a co-processor and its own operating system, with which it executes control tasks independently of the PC processor. Integrated connections enable access to decentralized actuators and sensors.

design typeHard PLCSoft PLCCompact PLCSlot PLC
notes• PLC hardware
solution • are started up quickly
• good real-time
behaviour • most important functions with expansion options
• PLC software solution
• more convenient to use
• slow start-up
• unstable at very high loads
• space-saving
• inexpensive
• suitable for small-scale tasks
• low space requirements
• they remain active thanks to their own power supply even if the PC fails
• simplified communication between PLC and PC thanks to visualization software

Modular PLC: A simple and individual solution for every machine system

Modular PLCs are not assemblies in the true sense but can be understood as a superordinate system. With modular controls, each individual functional component is housed in its own housing and on a separate control board. In this way, the PLC can be assembled individually using individual plug-in modules and easily expanded if required. An additional advantage is that in the event of a defect, only the respective assembly needs to be replaced and not the entire controller.


  • individual compilation of the required elements
  • Extension with the appropriate circuit board possible
  • for more complex regulation and control circuits

Some typical manufacturers and model series of modular PLCs


  • S7-300
  • S7-1200
  • S7-1500

Schneider Electric

⦁ M200s
⦁ M340
⦁ M580


⦁ CP1E
⦁ CP1H
⦁ CP1L

Allen Bradley

  • micro 820
  • micro 850
  • micro 870

Programming languages ​​for PLC

Various programming languages ​​can be found in the controls:

  • ST – Structured Text
  • FBD – function block language
  • LAD – Ladder Diagram
  • IL – Instruction List
  • SCL—Structured Control Language

FBD is popular with PLC beginners because it works with drag & drop and the behaviour of inputs and outputs is easy to understand. Sophisticated programming can be implemented with SCL. ST and IL are text-based, the other three graphical.

PLC timer | Industrial Control Person | Do you really know what a timer is?

Everything around us is evolving towards automation. Here, our PLC (Programmable Logic Controller) has played a big role. In PLC automation, there are different specifications of different types of PLC programming instructions for us to use. Among these PLC ladder diagram (LD) programming instructions, the timer instruction is one of the most important instructions, and it plays a very important role. This time, I will describe the PLC timer in detail through programming instructions and functions.

Let’s start from the beginning.

What is a PLC timer?

  • The basic internal circuit of PLC timer 1. Input and output module 2. Power supply module 3. Internal timer circuit 4. Timer digital display
  • What is the type of PLC timer? 1. Turn on the delay timer (TON) 2. Turn off the delay timer (TOFF) 3. Retentive on/off timer (RTO)
  • Timer instruction address of multiple SCADA brands 1. Address for ABB PLC 2. Address for AB (Rockwell) PLC 3. Address for Siemens PLC 4. Address for Delta PLC 5. Address for Mitsubishi PLC
  • Example based on PLC timer instruction
  • What are the applications of timer instructions?

What is a PLC timer?

The PLC timer is an instruction to control and operate equipment within a specific period of time. Using a timer, we can perform any specific operation within a specific period of time. We can set up time-based activities with the help of PLC programming timer instructions. Each PLC has different timer functions. Timer instructions are used to provide programming logic and decide when to open or close the circuit. It has normally open (NO) or normally closed (NC) contacts. Let us see here the representation of the input and output timers NO and NC contacts in LD programming. The timer output contacts are displayed in coil form, box form or rectangular form. In AB and Siemens PLC, it is represented by a box shape. If you want to perform work or equipment activities within a specific time frame, you must be familiar with timers. For this, we must learn the I/O timer instructions used to write PLC programs. In Ladder Diagram (LD) PLC programming, we can set the PLC timer from milliseconds (ms) to hours (hr).

Let us look at the internal circuit of the timer.

The basic internal circuit of the HMI timer

Now, we are looking at the internal timer circuit of the PLC. The operation of the timer circuit is based on four main parts.

Each internal part of the timer circuit has various functions. This is how they are connected and constructed in a given graph.

The following are some basic terms we need to know about timers used in PLCs.

1. Input and output modules

The module that interacts with the input signal is called the input module. The input module needs to be connected to the timer circuit to provide input signals.

The module that interacts with the output signal is called the output module. The output module needs to be connected to the timer circuit.

2. Power module

The power module provides power for the normal operation of the timer circuit. It can be connected to an AC voltage source (for example, 120, 230 V AC) or a DC voltage source (for example, 5, 12, 24 V DC).

3. Internal timer circuit

The timer circuit performs set and reset functions.

If the auxiliary power supply is “on”, the timer will provide instantaneous input pulses for setting and reset operations.

4. Timer digital display

The digital timer displays the set and elapsed timing values.

For automation, these values ​​can be displayed within a few milliseconds (ms). This will make it easy to track your automation system.

What is the type of PLC timer?

For ladder diagram programming, the classification of the PLC programming timer is-

1. Turn on the delay timer (TON)

A delay timer (TON) is a programming instruction used to start instantaneous pulses within a set period of time.

Let us look at the simple structure of AB PLC delay timer programming instructions.

2. Turn off the delay timer (TOFF)

The time delay (TOF) timer is a PLC programming instruction used to turn off the output or the system after a certain period of time.

See here, the basic structure of AB PLC close delay timer programming instruction.

3. Retentive on/off timer (RTO)

RTO main function for saving or storing settings (cumulative) time.

RTO will be used when the cascade status changes, power loss or any interruption in the system.

In AB PLC, the retentive timer instructions are as follows.

We can briefly understand various types of PLC timers through examples.

Timer instruction address of multiple PLC brands

We have seen that three timers provide a delay function to control the operation of the PLC. The timer handles four main values.

  • Timer address
  • default value
  • Basic timer value
  • Cumulative value

Each timer instruction has three very useful status bits. These bits are…

  1. Enable bit (EN)
  2. Timer timing position (TT)
  3. Done bit (DN).

In AB and Siemens PLC, the output bit is usually called the “done bit” of the timer. And it indicates that the timer has reached its preset time.

1. Addressing ABB PLC

In ABB HMI programming, we can simply program the I/O timer address of the ladder diagram. We can set the timer value between ” T0 ” and ” T255 “.

You can see the I/O contact representation diagram above.

2. Addressing AB (Rockwell) PLC

For AB PLC, the address range of the timer is from ” T4:0″ to ” T4:255 “.

Among them, T4 is the file type.

The addressing format of the timer instruction with three status bits.

  1. The address range of the enable bit (EN) is from’T4: 0 / EN’ to’T4: 255 / EN’.
  2. The addressing range of the timer timing bit (TT) is from’T4:0 / TT’ to’T4: 255 / TT’.
  3. The completion bit (DN) address ranges from’T4:0 / DN’ to’T4: 255 / DN’.

3. Address of Siemens PLC

In Siemens, five types of timers can be used to write LD programs.

  • Pulse timer (S_Pulse)
  • Pulse extension timer (S_PExT)
  • Delay timer (S_ODT)
  • Delay extended timer (S_ODTS)
  • Off delay timer (S_OffDT)

The general block diagram of the timer (in Siemens ),


S-the set value or signal of the timer

TV-time variable. It is used to store time values ​​in the following form:

You can enter a time value between 1 and 9990 seconds.

R-timer reset value

Q-timer output

BI-current time in binary code

BCD-current time (binary decimal code)

4. Addressing Delta PLC

For WPLSoft software (Delta ), you can use timer addressing, ranging from ‘ T0′ to’ T127 ‘.

In Delta PLC, enter the timer address as shown in the general representation (T0, T1, … T127). And the form of the output coil is


“T0” is the timer address, “K” is a constant item

Block diagram of Delta PLC timer:

For Delta PLC, the timer will start for 10 seconds. It should be written in the form of “T0 K100”.

5. Address of Mitsubishi PLC

Both Mitsubishi PLC and Delta HMI use the same timer addressing format.

Example based on PLC timer instruction

The most basic and practical example is the use of PLC to automatically control traffic signals.

After a certain (fixed) time, each side signal must be turned on and off. Only one traffic light should be turned on at a time.

A simple PLC timer can be used to implement this logic.

What are the applications of timer instructions?

These are some basic applications of timers that can be used in the PLC automation environment.

  1. Used for delayed action
  2. Used to run or stop operations according to user commands.
  3. The RTO timer helps to record or maintain intermediate time values.

All this is related to the PLC timer. This is a topic that can be said a lot. I just talked briefly this time. If you have any questions, please feel free to ask in the comments.

If you want a detailed application of the PLC , we will talk about it later.

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