PLC Timer Instructions and Types
In this article we will learn PLC timer instruction and their types. A PLC timer is an instruction used in ladder logic programing. When added to a ladder diagram it introduces a time delay when a specific event occurs. In its most basic form, a PLC timer has a start input, a preset time input and a done output.
For basic instruction in plc use Ladder Logic Programming Instructions – Part 1
What is the PLC Timer?
PLC timer instructions are internal instructions that delay input and output signals in a PLC program. They can be used to activate or deactivate a device after a set amount of time.
A timer is one of the most essential and useful entity.
You can set time-based activity with the help of the PLC programming timer instruction. Every PLC having different timer functions.
The timer instruction is used to provide programming logic and to decide when to turn on or off the circuit. It has both normally open (NO) or normally closed (NC) contact.
Let’s see here, the representation of the input and output timer NO and NC contact in LD programming.
As per applications and industrial use, there are different types of timers that are used.
Types of PLC Timers
Different types of PLC timers exist based on how they implement the time delay within the timing instruction. The 3 types of PLC timers, as defined in IEC 61131-3 International PLC Programming Standard, are:
- ON-Delay Timer (TON)
- OFF-Delay Timer (TOF)
- Retentive On/Off Timer (RTO)
1. ON Delay Timer (TON)
In a PLC the ON delay timer is commonly known as a TON timer. The operation of the ON delay timer is a reflection of its name. You see, the ON Delay Timer (TON Timer) only turns its output ON after a preset time DELAY. In other words, the output is DELAYED before it turns ON.
For example, the TON instruction in a PLC is used in Allen Bradley, Siemens, ABB and Unitronics PLCs. Whereas, the Omron, Mitsubishi, Automation Direct, Delta and Idec PLCs do not use the TON notation for their ON delay timer instruction.
2. OFF Delay Timer (TOF)
In a PLC the OFF-delay timer is commonly known as a TOF timer. The operation of the OFF-delay timer is a reflection of its name. You see, the OFF-Delay Timer (TOF Timer) only turns its output OFF after a preset time DELAY. In other words, the output is DELAYED before it turns OFF.
For example, the TOF instruction in a PLC is used in Allen Bradley, Siemens, ABB, Unitronics, Schneider and Emerson PLCs. Whereas the Automation Direct and Idec PLCs do not use the TOF notation for their OFF-delay timer instruction. In fact, the Omron, Mitsubishi and Delta PLCs do not have an OFF-delay timer instruction at all.
3) PLC Retentive (Accumulating) Timer
In a PLC the Retentive Timer is commonly known as a RTO timer (Retentive Timer ON). The operation of the Retentive Timer is a reflection of its name. You see, it RETAINS the amount of time that the input is active for and only turns the output ON once the total retained time is equal to the preset time value.
The PLC Retentive Timer is also known as an Accumulating Timer in some PLC brands. Some examples of PLCs with accumulating timer instructions are Omron with TTIM, Unitronics with TA, Automation Direct with TMRA instruction. Whether the terminology used is retentive timer or accumulating timer their operation is essentially the same.
Frequently ask questions (FAQ)
1. What is the use of timer in PLC?PLC timers are internal PLC instructions that can be used to delay input and output signals in the PLC program. These timers operate like relay timers but you cannot hold a PLC timer in your hand and they do not need to be connected to wires to operate
2. Why are timers used?
Timers are devices that can switch on or off an electrical circuit after a preset or variable time interval. They are widely used in electrical controls for various applications, such as lighting, heating, cooling, motor control, automation, and safety.
3. What is the function of the timer?
Timers are peripheral devices that can be used to set the interval between events. In embedded systems design, the ability to precisely set the timing between events is extremely common. Timers are used to in multi-threaded operating systems to determine how long a task is active before swapping to a new task.
4. What is pulse timer in PLC?
In a PLC the Pulse Timer is commonly known as a PT timer. The Pulse Timer creates an output PULSE with a pulse duration equal to that of the preset time value. In simple terms, the TP timer output is ON when the timer is running and is OFF all other times.
5. What is the role of the timer?
Members rely on the timer to pace speeches and practice adhering to a time frame. The timer is also responsible for tracking every part of the meeting agenda. To fulfill the role of the timer, you must know each presenter's speech length.
PLC Related More Articles
- PLC Programming - Basic Knowledge
- Ladder Logic Programming Instructions – Part 1
- Ladder Logic Programming Instructions – Part 2
- PLC Interview Questions and Answers for Beginners
- Difference between PLC and Relay Based Controller
- PLC Input and Output —Types and Examples
- PLC Sensor Types and their Selection Process
