TimersTimers

• PLC timers are output instruction that PLC timers are output instruction that provides the same function as mechanical provides the same function as mechanical timing relay.timing relay.

• We have two kinds of timers: ON delay & We have two kinds of timers: ON delay & OFF delay OFF delay

• ON delayON delay provides the delay when the provides the delay when the relay is energized.relay is energized.

• OFF delayOFF delay provides the delay when the provides the delay when the relay is de energized. relay is de energized.

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.

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.

OFF delay symbolsOFF 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.

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.

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

On-Delay Timer:On-Delay Timer:

• Use the TON instruction to turn an output on or Use the TON instruction to turn an output on or off after the times has been on for a preset time off after the times has been on for a preset time interval. The TON instruction starts accumulating interval. The TON instruction starts accumulating time when the rung goes true, and continues until time when the rung goes true, and continues until one of the following happens.one of the following happens.

• the accumulated value equals its preset value the accumulated value equals its preset value • the rung goes false the rung goes false • a reset instruction resets the timer a reset instruction resets the timer • The processor resets the accumulated value The processor resets the accumulated value

when the rung conditions go false, regardless of when the rung conditions go false, regardless of whether the timer timed out or not.whether the timer timed out or not.

Timer Words and Bits:Timer Words and Bits:

• A timer element contains the following addressable A timer element contains the following addressable words and bits that provide information concerning words and bits that provide information concerning the timer:the timer:

• * Preset Value (.PRE):* Preset Value (.PRE): the value that the timer the value that the timer must reach to change the state of the done bit to 1.must reach to change the state of the done bit to 1.

• *Accumulated Value (.ACC): *Accumulated Value (.ACC): the number of time the number of time increments that have elapsed. This value can range increments that have elapsed. This value can range from 0 to +32,767.from 0 to +32,767.

• **Enable Bit (EN):Enable Bit (EN): a bit set when the rung goes true. a bit set when the rung goes true.• *Timer Timing Bit (TT): *Timer Timing Bit (TT): a bit set while the timer is a bit set while the timer is

incrementing.incrementing.• **Done Bit (DN):Done Bit (DN): a bit that sets when the a bit that sets when the

accumulated value reaches the preset.accumulated value reaches the preset.

Time BaseTime Base::

• The time base can be selected at 1.0 or The time base can be selected at 1.0 or 0.01, depending on the accuracy required:0.01, depending on the accuracy required:

• The amount of time that you can time-out The amount of time that you can time-out on a timer is limited by the total value that on a timer is limited by the total value that a preset address (16 bits) can hold a preset address (16 bits) can hold (32,767). (32,767).

• If the time base is 0.01 seconds, this limits If the time base is 0.01 seconds, this limits the preset time to 0.01 x 32,767 = 327 the preset time to 0.01 x 32,767 = 327 seconds. seconds.

Off-Delay TimerOff-Delay Timer

• Description:Description:• Use the TOF instruction to Use the TOF instruction to

turn an output on or off after turn an output on or off after its rung has been off for a its rung has been off for a preset time interval. The TOF preset time interval. The TOF instruction starts instruction starts accumulating time when the accumulating time when the rung goes false and continues rung goes false and continues timing until one of the timing until one of the following conditions occur:following conditions occur:

• the accumulated value equals the accumulated value equals its preset value its preset value

• the rung goes true the rung goes true • a reset instruction resets the a reset instruction resets the

timer timer

The processor resets the accumulated value when the rung conditions go true, regardless of whether the timer timed out or not.

If the processor loses If the processor loses power:power:• Change to program mode, or the processor Change to program mode, or the processor

fault interrupts the TOF instruction before it fault interrupts the TOF instruction before it reaches the preset value, the following reaches the preset value, the following occurs:occurs:

• timer enable (.EN) bit remains reset timer enable (.EN) bit remains reset • timer timing (.TT) bit remains set timer timing (.TT) bit remains set • timer done (.DN) bit remains set timer done (.DN) bit remains set • accumulated (.ACC) value remains the same accumulated (.ACC) value remains the same • 2) Then if you switch to Run mode or Test 2) Then if you switch to Run mode or Test

mode, the following happens:mode, the following happens:

Retentive Timer:Retentive Timer:

Retentive Timer:

• Use the RTO instruction to turn an output on or off Use the RTO instruction to turn an output on or off after its timer has been on for a preset time interval. after its timer has been on for a preset time interval. The RTO instruction lets the time stop and start The RTO instruction lets the time stop and start without resetting the accumulated value.without resetting the accumulated value.

• The RTO instruction begins timing when it rung goes The RTO instruction begins timing when it rung goes true. As long as the rung remains true, the timer true. As long as the rung remains true, the timer updates the accumulated value each program scan, updates the accumulated value each program scan, until it reaches the preset value. The RTO instruction until it reaches the preset value. The RTO instruction retains its accumulated value even if one of the retains its accumulated value even if one of the following occurs:following occurs:

• the rung goes false the rung goes false • you change to Program mode you change to Program mode • the processor faults of loses power the processor faults of loses power • the associated SFC step goes inactive the associated SFC step goes inactive

• When the processor resumes operation or the rung When the processor resumes operation or the rung goes true, timing continues from the retained goes true, timing continues from the retained accumulated value. By retaining its accumulated accumulated value. By retaining its accumulated value, retentive timers measure the cumulative value, retentive timers measure the cumulative period during which its rung is true.period during which its rung is true.

• Important:Important: To reset the retentive timer’s To reset the retentive timer’s accumulated value and status bits after the RTO rung accumulated value and status bits after the RTO rung goes false, you must program a reset instruction RES goes false, you must program a reset instruction RES with the same address in another rung.with the same address in another rung.

• Using Status Bits:Using Status Bits:• Examine status bits in the ladder program to trigger Examine status bits in the ladder program to trigger

some event. The processor changes the states of some event. The processor changes the states of status bits when the processor runs this instruction. status bits when the processor runs this instruction. You address status bits by mnemonic.You address status bits by mnemonic.

If the processor loses If the processor loses power:power:• Change to Program mode, Change to Program mode,

or processor fault interrupts or processor fault interrupts the RTO instruction, the the RTO instruction, the following occurs:following occurs:

• timer enable (.EN) bit timer enable (.EN) bit remains set remains set

• timer timing (.TT) bit timer timing (.TT) bit remains set remains set

• accumulated (.ACC) value accumulated (.ACC) value remains the same remains the same

• Then when you switch back Then when you switch back to Run mode or Test mode, to Run mode or Test mode, the following happens:the following happens: