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Timer Instructions Overview
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Timer Applications
Delay motors from starting at the same time
Keep track of downtimeOpen a valve for a given time
(e.g.,10 seconds)Measure the cycle time of a process
Practical ExamplesKeep track of total time required to
make a batch of a productTurn mixing motor off 2 minutes after
the switch is closedHold clamp on for 25 seconds after glue
is appliedOpen a valve 27 seconds after a switch
is turned on. If interrupted, the valve should close and the time be reset to 0.
Categories of Timer Instructions
TON (Timer on delay)Timer remains true for a preset time interval
TOF (Timer off delay)Timer remains false for a preset time
interval
RTO (Retentive timer on delay)Timer remains on for a preset time interval
Parts of a Timer Instruction
Word 0: Control word Enable bit(.EN) -- is set when the rung is
true Timer Done Bit (.DN)-- changes state
when accumulated value reaches a preset value
Timer Timing bit (.TT)-- is set while the timer is incrementing its accumulated value
Parts of a Timer Instruction (cont.)
Word 1: Preset Value (.PRE) the value the timer must reach before the
controller sets the DN bit. When ACC = PRE, the DN bit is set. PRE can be used to control an output device.
Word 2: Accumulated Value (.ACC) the time elapsed since the timer was last
reset. When ACC is enabled, the timer updates it continually.
Parameters to Enter
Accumulated value (.ACC)Preset value (.PRE)Time base - the duration of each
time base unitDuration of timer = (preset value - accumulator value) * time base
TON Instruction
TON stands for...
TIMER DELAY ONTime delay period is from the instant
the rung goes TRUE until the DN bit is turned on
TON can be used to...
Create a short pulse at the beginning of a longer input condition;
Delay the start of a function for a defined period of time from the start of some other function
TON Ladder Diagram
TON operation
When the rung goes TRUE, the EN and TT bits go on.
When the accumulated value (ACC) reaches the preset value (PRE), TT bit goes off EN bit stays on DN bit turns on
TON operation (cont.)
When the rung goes from TRUE to FALSE and the accumulated value (ACC) is less than the preset value (PRE) EN and TT bits turn off Accumulated value is reset to 000
Timer reset instructions can be used to clear ACC
TON EN bit
Is set when rung conditions are TRUERemains set until rung conditions go
FALSE
TON TT bit
Is set when rung conditions are TRUE
AND ACC < PRE
Remains set until rung conditions go FALSE
TON DN bit
Is set when ACC = PRERemains set until rung conditions go
FALSE
Worked Out Problems
Turn on L1 if sw1 is closed for 12 seconds.
When sw1 is closed, turn on motors 10, 11, and 12. Turn off motors 10, 11, and 12 at five-second intervals when sw1 is opened.
TOF Instruction
TOF stands for...
TIMER DELAY OFFTime delay period is from the instant
the rung goes TRUE to FALSE until the DN bit is turned off
TOF can be used to...
Create longer output functions derived from short input functions;
Generate a short pulse at the end of a long input function
TOF Timer Instruction
TOF instruction begins to count time base intervals when the rung makes a true-to-false transition.
As long as rung conditions remain false, the timer increments its accumulated value (ACC) each scan until it reaches the preset value (PRE).
The controller resets the accumulated value when rung conditions go true regardless of whether the timer has timed out
Sheng-Jen (“Tony”) Hsieh, Texas A&M University
Using the TOF Status Bits
Sheng-Jen (“Tony”) Hsieh, Texas A&M University
Where, the bit is set means set to 1
TOF Ladder Diagram
TOF operation
When the rung goes TRUE, the EN and DN bits go on.
When the accumulated value (ACC) reaches the preset value (PRE), DN bit goes off AC is reset to 000
TOF operation (cont.)
When the rung goes from TRUE to FALSE and the accumulated value (ACC) is less than the preset value (PRE) DN bit stays on Timer begins to accumulate time for the
delay period TT bit goes on and stays on for preset time,
then resets
Timer reset instructions cannot be used to clear ACC
TOF EN bit
Is set when rung conditions are TRUERemains set until rung conditions go
FALSE
TOF TT bit
Is set when rung conditions are TRUE to FALSE
AND AC < PR
Remains set until rung conditions go TRUE again
TOF DN bit
Is set when rung conditions are TRUEWhen rung conditions go from TRUE
to FALSE and ACC < PRE Remains set until
ACC = PRE
RTO Instruction
RTO stands for...
RETENTIVE TIMER DELAY ONSimilar to TONTime delay period is from the instant
the rung goes TRUE until the DN bit is turned on (like TON)
RTO can be used for...
Maintenance functions;Diagnostic programs
RTO Timer Instruction
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 is a retentive instruction that lets the timer stop and start without resetting the accumulated value (ACC).
Sheng-Jen (“Tony”) Hsieh, Texas A&M University
Using the RTO Status Bits
Sheng-Jen (“Tony”) Hsieh, Texas A&M University
To reset the ACC value and status bits, you must program a reset (RES) instruction with the same address
Timer: RTO
RTO operation
(Like TON) When the rung goes TRUE, the EN and TT bits go on.
(Like TON) When the accumulated value (ACC) reaches the preset value (PRE), TT bit goes off EN bit stays on DN bit turns on
RTO operation (cont.)
When the rung goes from TRUE to FALSE and the accumulated value (AC) is less than the preset value (PR) EN and TT bits turn off Accumulated value is NOT reset
Timer reset instructions must be used to clear AC
RTO EN bit
(like TON) Is set when rung conditions are TRUE
(like TON) Remains set until rung conditions go FALSE
RTO TT bit
(like TON) Is set when rung conditions are TRUE
AND AC < PR
(like TON) Remains set until rung conditions go FALSE
OR when the DN bit is set
RTO DN bit
(like TON) Is set when AC => PRRemains set until the appropriate
RES instruction is enabled
Close this window and then select items 4, 5, 6 TON, TOF, and RTOSimulation.