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Timers

Timers. PLC timers are output instruction that provides the same function as mechanical timing relay. We have two kinds of timers: ON delay & OFF delay ON delay provides the delay when the relay is energized. OFF delay provides the delay when the relay is de energized.

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Timers

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  1. Timers

  2. PLC timers are output instruction that provides the same function as mechanical timing relay. • We have two kinds of timers: ON delay & OFF delay • ON delay provides the delay when the relay is energized. • OFF delay provides the delay when the relay is de energized.

  3. NOTC (normally open, timed close contact)- contact is open, when relay is de energized; when relay is energized, there is a time delay in closing.

  4. NCTO (normally closed, timed open contact)-contact is closed when relay coil is de energized. When relay is energized, there is a time delay in opening.

  5. OFF delay symbols: NOTO (Normally open, timed open contacts)- contact is normally open, when relay coil is de energized when relay coil is energized contact closes instantly. When relay coil is de energized, there is a time delay before the contact opens.

  6. NCTC (normally closed, timed close contact)- contact is normally closed when relay coil is de energized. When the relay coil is energized, contact open instantly. When the relay coil is de energized, there is a time delay before the contact closes.

  7. There are two methods to show the time within the PLC ladder diagram:

  8. On-Delay Timer: • Use the TON instruction to turn an output on or off after the times has been on for a preset time interval. The TON instruction starts accumulating time when the rung goes true, and continues until one of the following happens. • the accumulated value equals its preset value • the rung goes false • a reset instruction resets the timer • The processor resets the accumulated value when the rung conditions go false, regardless of whether the timer timed out or not.

  9. Timer Words and Bits: • A timer element contains the following addressable words and bits that provide information concerning the timer: • * Preset Value (.PRE): the value that the timer must reach to change the state of the done bit to 1. • *Accumulated Value (.ACC): the number of time increments that have elapsed. This value can range from 0 to +32,767. • *Enable Bit (EN): a bit set when the rung goes true. • *Timer Timing Bit (TT): a bit set while the timer is incrementing. • *Done Bit (DN): a bit that sets when the accumulated value reaches the preset.

  10. Time Base: • The time base can be selected at 1.0 or 0.01, depending on the accuracy required: • The amount of time that you can time-out on a timer is limited by the total value that a preset address (16 bits) can hold (32,767). • If the time base is 0.01 seconds, this limits the preset time to 0.01 x 32,767 = 327 seconds.

  11. Off-Delay Timer • Description: • Use the TOF instruction to turn an output on or off after its rung has been off for a preset time interval. The TOF instruction starts accumulating time when the rung goes false and continues timing until one of the following conditions occur: • the accumulated value equals its preset value • the rung goes true • a reset instruction resets the timer The processor resets the accumulated value when the rung conditions go true, regardless of whether the timer timed out or not.

  12. If the processor loses power: • Change to program mode, or the processor fault interrupts the TOF instruction before it reaches the preset value, the following occurs: • timer enable (.EN) bit remains reset • timer timing (.TT) bit remains set • timer done (.DN) bit remains set • accumulated (.ACC) value remains the same • 2) Then if you switch to Run mode or Test mode, the following happens:

  13. Retentive Timer: Retentive Timer:

  14. Use the RTO instruction to turn an output on or off after its timer has been on for a preset time interval. The RTO instruction lets the time stop and start without resetting the accumulated value. • The RTO instruction begins timing when it rung goes true. As long as the rung remains true, the timer updates the accumulated value each program scan, until it reaches the preset value. The RTO instruction retains its accumulated value even if one of the following occurs: • the rung goes false • you change to Program mode • the processor faults of loses power • the associated SFC step goes inactive

  15. When the processor resumes operation or the rung goes true, timing continues from the retained accumulated value. By retaining its accumulated value, retentive timers measure the cumulative period during which its rung is true. • Important: To reset the retentive timer’s accumulated value and status bits after the RTO rung goes false, you must program a reset instruction RES with the same address in another rung. • Using Status Bits: • Examine status bits in the ladder program to trigger some event. The processor changes the states of status bits when the processor runs this instruction. You address status bits by mnemonic.

  16. If the processor loses power: • Change to Program mode, or processor fault interrupts the RTO instruction, the following occurs: • timer enable (.EN) bit remains set • timer timing (.TT) bit remains set • accumulated (.ACC) value remains the same • Then when you switch back to Run mode or Test mode, the following happens:

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