PLC learning Series 10: PLC timers

What is PLC Timers?

PLC timer is an element in ladder logic programming taken from electrical systems. Timers are devices that count time increments.

The timer creates a delay in both PLC programming and in relay boards on connection or instantaneous disconnection.

Simply, when the input is activated the timer starts its operation keeping track of the time. When this time exceeds the programmed time then the timer activates its output.

You could set the PLC timer from millisecond (ms) to an hour (hr) time span in the programming of the Ladder Diagram (LD) PLC.

There are three types of PLC timers:

  • ON delay timer (TON)
  • OFF delay timer (TOFF)
  • Retentive Timer (RTO)

ON delay Timer:

On delay means that once a timer has received a power on signal, its output will change state after a predetermined delay.

The timer will be ON when a start input siganl is received, and when a preset timing is reached, the signal state of the output changes from 1 to 0.

The signal status of the output adjusts from 0 to 1 when the preset time is reached by changing the RLO (Result of logic operation) from 0 to 1(ON) at the starting input.

For example, When input S1 is activated, timer TR1 starts counting. When pre programmed time have passed with input S1 activated, the TR1 output is activated, energizing output PL1.

OFF delay timer:

The OFF delay means that it will change state a predetermined time after the timer has received the shutdown signal.

When the off-delay timer (True) is energized in PLC programming, it immediately turns its output on. The display will be ‘ on ‘ until the setting time is reached.

The timer begins to work when the rung signal is false The timer continues to accumulate times until the accumulated value becomes equal to the preset value. When the accumulated value is equal to the preset value, the output turns off.

Retentive Timer On (RTO):

The RTO’s main function is used to keep or store the (accumulated) set time. It is used when rung state change, power loss or any device interruption occurs.

When its rung goes from false to true, the Accumulate value of the retentive timer counts from where it stopped the last time.

The reset instruction will be activated when its rung is true and resets the timer according to the address assigned to the reset instruction.

The timer’s power loss after reaching its preset value does not affect the contacts ‘ state. The retentive timer must be reset with a different signal for the reset time.

Timer address for different PLC brand:

The output bit is often referred to as the ‘ Completed bit ‘ of the timer in the AB or Siemens PLC. And it indicates that the clock has reached its preset time.

Addressing for ABB PLC

We can simply write the ladder diagram’s I / O address in the ABB PLC. We can set the value in T0 to T255 ranges.

Addressing for Siemens PLC

In the Siemens, LD program can be written with the five types of timers.

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

Addressing for AB (Rockwell) PLC

The timer has the address between T4:0 and T4:255 for the AB PLC. Where T4 is the type of file. Addressing format with the three status bits for timer instruction.

Addressing for Delta PLC & Mitsubishi:

You can use timer addressing from ‘ T0 to T127 ‘ for the WPLSoft program. The input timer address is shown as a general representation in Delta PLC (T0, T1, …….. T127). And output coil is written as

Where ‘ T0 ‘ is the address of the timer and ‘ K ‘ is the constant term.

PLC learning series

PLC learning Series I: What is PLC? Functions of PLC
PLC learning series 2: PLC programming languages
PLC learning series 3: PLC Architecture and wiring of PLC
PLC learning series 4 : How a ladder logic diagram works? How to read ladder logic ?
PLC learning series 5: 8 Rules for ladder diagram programming
PLC learning series 6: PLC process Scan basics
PLC learning series 7: Functional Block Diagram program, symbols
PLC learning series 8: Instruction List programming
PLC learning series 9: PLC selection criteria.
PLC learning Series 10: PLC timers
PLC learning series 11 : How to interface PLC with SCADA?
PLC learning series 12: Troubleshooting PLC
PLC learning Series 13: Counters in PLC


Instrumentation Engineer

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