Pulse Timer Instruction in PLC Programming

Let’s look at how the pulse timer command in Siemens PLC programming works.

Pulse Timer in PLC

Block Description

Pulse Timer:

When the Timer gets a positive pulse at the set input (S), it enables the Q output for the duration of the timer’s preset setting. In order to enable the timer, the signal must be changed. The timer will continue to run until the input sign (S) is set to “1” or the accumulator value reaches the preset value. As long as the timer is operating, the signal condition at the output (Q) is “1.” If the value of input (S) is changed from “1 to 0,

The timer reset input allows you to restart a running timer. The value of the accumulator is reset to zero. The value of the accumulator can be accessed via the timer’s BI and BCD outputs.

Pulse Extended Timer:

When the Timer gets a positive pulse at the set input (S), it enables the Q output for the duration of the timer’s preset setting. In order to enable the timer, the signal must be changed.

As long as the timer is operating, the signal condition at the output (Q) is “1.” If the input (S) is changed from “1 to 0” before the time interval has passed, the timer continues to run and the output (Q) is enabled until the time interval has passed.

The timer reset input allows you to restart a running timer. The value of the accumulator is reset to zero. The value of the accumulator can be accessed via the timer’s BI and BCD outputs.

Ladder Logic

Case 1: When Input set (S) goes ON for the first time

Network 1:

When input I0.0 is pressed, a positive pulse is sent to the S input of the timer (T0), causing the timer to begin operating for 20 seconds, the time specified in the preset setting (TV=ST#20s). Timer (T0) output (Q0.0) is enabled for 20 seconds.

Network 2:

When input I0.2 is pressed, a positive pulse is sent to the S input of the timer (T1), causing the timer to begin operating for 20 seconds, the duration specified in the preset setting (TV=ST#20s). Timer (T1) output (Q0.1) is enabled for 20 seconds.

Case 2: When Input set (S) goes OFF before time interval (20s) get elapsed

Network 1:

When the timer (T0input )’s (I0.0) is shut off before the time interval has elapsed, the timer will stop running and the output (Q0.0) will switch off.

Network 2:

When the timer (T1input )’s (I0.2) is switched off before the time interval has elapsed, the timer will continue to run and the output (Q0.1) will remain ON until the time interval has elapsed.

Case 3: When Reset input (R) goes ON before time interval (20s) get elapsed

Network 1:

When the Reset Input (I0.1) to the timer (T0) is pressed, the timer stops running and the output is disabled (Q0.0).

Network 2:

When the Reset Input (I0.3) to the timer (T1) is pressed, the timer stops running and the output is disabled (Q0.1).

As we all know time plays an important role for most of the sequential tasks in industrial, domestic and commercial applications. PLC timer instructions can give independence from hard wiring to mechanical and electromagnetic timer relays. In this blog, we are going to look at a brief overview of PLC timer instructions along with practical examples.

PLC Timer Instructions

Timer plays an important role to control the operation for a specific period. As a very basic example, we need a timer to control the transition from star to delta function in a star-delta electrical motor starter.

Electromagnetic Time-Delay Relay vs. PLC Timer Function

In a conventional control relay system, as soon as voltage is applied to the coil, the contact changes. Whereas in a time-lapse relay, the contact changes after a predetermined time when voltage is applied to the coil.

The time-lapse relay is very easy to use. All you have to do is apply voltage to the coil and wait for a predetermined time to change contacts. This is the block diagram of the time-lapse relay:

There are different types of time-lapse relays available in the market:

on-delay timer
off-delay timer
on/off delay timer
on-delay timer relay
In an on-delay timer, timing starts when voltage is applied to the coil of the timer. As the time elapses, the contact turns and keeps changing until the voltage is removed from the coil.

off-delay timer relay
In an off-delay timer when voltage is applied to the coil nothing happens. When the input/trigger switch is closed, the output contact is switched on and timing is triggered. The output contact keeps changing until the time has elapsed or the voltage is removed from the coil.

on/off delay timer relay
In On/Off Delay Timer, we have functionalities of both On-Delay Timer and Off-Delay Timer. This type of time delay relay also has a repeat-cycle functionality.

Advantages of PLC Timer over Electromagnetic Time-Delay Relays

Time value and timer function can be changed very easily without any hardware changes just by changing the program. PLC timer requires no maintenance. We can use multiple contacts of the PLC timer to program, no additional standard control relays are needed to forward the logic. Users can easily implement logic using PLC timer as per requirement.

In Siemens PLC, S_PULSE (Pulse Timer) S_PEXT (pulse-extended timer) S_ODT (on-delay timer) S_ODTS (Perceptual On-Delay Timer) S_OFFDT (off-delay timer) At Allen Bradley PLC, Tones (on-delay timer) TOF (off-delay timer) RTO (Retention On-Delay Timer) The addressing is different for different brands of PLC, but the logical operations and functions are very similar.

Each timer block consists of bits such as S, R, Q and words such as Tv, Bi and Bcd.

S set the bit to trigger the timer block

R reset the bit to reset the timer block.

Q The status bit of the timer block.

Preset time value of TV timer block.

BI The remaining time value in an integer format.

BCD remaining time value in BCD format.

PLC timer example

There are many examples in which PLC timer instructions are used. Out of all this, I am giving you examples of some of the most useful logic like,

traffic control signs
zero speed sensor detection
Tool Running Position Interlocked
closed loop control
equipment tripping command
Hourly and daily data logging
To calculate the flow rate
PID control
Calculate motor running hours