PLC Software Controller Timer

Timers in PLC software controller are special operands the define time delays in relay logic systems. In PLC, time functions are fixed components within the central processing units. The number of timers vary based on product requirements and manufacturer preferences. However, time delays can occur within a few milliseconds to a number of hours. Note that, timers have a predefined register value representing its maximum count and it is set using a program or software. PLC software controller utilize different types of timers as explained below.

On delay timer
When an input signal is triggered and turns on, the output signal also turns on but after a short delay. However, when the input signal goes off, the output signal goes off immediately as well. As such, if the input signal turns on then off within the set delay time, the output signal will not record any changes. It remains off despite the input signal recording and brief on then off signal. Meaning that, under the on delay timer, output is not recorded unless the input pulse duration extends beyond the predetermined delay time.

Off delay timer
Under the off delay timer, the output signal turns on immediately the input signal turns on. However, the output signal goes off after a certain delay period after the input signal goes off. Therefore, if the input signal records an instant on then off, without hitting the predefined delay period, the output signal remains unchanged under the on condition despite the input going off. Meaning that, the output delay cannot be observed unless the input pulse occurs in a greater width than the predefined delay period.

Fixed pulse width timer
The output signal turns on immediately the input signal records an on condition. The output condition retains the on condition for a fixed duration before recoding an off. Note that, the input pulse width and the output pulse width are independent of each other.

Retentive timer
The retentive timer is implemented internally within a register. As such, once the input signal records an on condition, the internal counter begins recoding time until the input signal goes off and it holds the recorded value until application of the next input pulse. It then starts counting as per the value recorded in the register. The output goes on only after the counter achieves the terminal count. Hence the name retentive timer.

Non-retentive timer
Just like the retentive timer, the non-retentive timer is implemented internally within a register. The internal counter begins recording time immediately the input goes on and once it goes off the counter resets to zero. Thus, the name non-retentive timer. Note that, the output goes on once the counter achieves a predetermined terminal count.

The counting function for the different timers’ works as a hardware counter and it is a fixed central processing unit component. The number of counters may vary based on the PLC software controllers in use. But, the counting range falls between 0 and 999.