1. Timer Instructions Overview
Instructions: Use the Page Up and Page Down keys to navigate through this presentation. When you are done viewing the presentation, close this window.

2. Timers: Applications
Delay motors from starting at the same time when a switch is closed (TON Done bit)
Keep track of downtime – machine is up most of the time and down sometimes (RTO ACC bit)
Open a valve for a given time after a switch is closed (e.g.,10 seconds)
Measure the cycle time of a process (RTO ACC bit)
Fall S. Hsieh

3. Examples
Keep track of total time required to make a batch of a product (RTO ACC bit)
Turn mixing motor off 2 minutes after the switch is closed (TON – Timing bit)
Hold clamp on for 25 seconds after glue is applied after a switch is closed then open (TOF – Timing bit)
Open a valve 27 seconds after a switch is turned close. If interrupted, the valve should close and the time be reset to 0. (TON – Timing bit)
Fall S. Hsieh

4. 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

5. 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

6. 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.

7. Parameters to Enter Accumulated value (.ACC) Preset value (.PRE)
Time base - the duration of each time base unit
Duration of timer =
(preset value - accumulator value) * time base

8. TON Instruction

9. TON stands for... TIMER DELAY ON
Time delay period is from the instant the rung goes TRUE until the DN bit is turned on

10. 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

11. TON Ladder Diagram

12. 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

13. 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

14. TON EN bit Is set when rung conditions are TRUE
Remains set until rung conditions go FALSE

15. TON TT bit Is set when Remains set until rung conditions are TRUE AND
ACC < PRE
Remains set until
rung conditions go FALSE

16. TON DN bit Is set when ACC = PRE
Remains set until rung conditions go FALSE

17. 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.

18. TOF Instruction

19. TOF stands for... TIMER DELAY OFF
Time delay period is from the instant the rung goes TRUE to FALSE until the DN bit is turned off

20. 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

21. Sheng-Jen (“Tony”) Hsieh, Texas A&M University
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

22. Using the TOF Status Bits
Sheng-Jen (“Tony”) Hsieh, Texas A&M University
Using the TOF Status Bits
Where, the bit is set means set to 1

23. TOF Ladder Diagram

24. 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

25. 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

26. TOF EN bit Is set when rung conditions are TRUE
Remains set until rung conditions go FALSE

27. TOF TT bit Is set when Remains set until
rung conditions are TRUE to FALSE AND
AC < PR
Remains set until
rung conditions go TRUE again

28. TOF DN bit Is set when rung conditions are TRUE
When rung conditions go from TRUE to FALSE and ACC < PRE
Remains set until
ACC = PRE

29. RTO Instruction

30. RTO stands for... RETENTIVE TIMER DELAY ON Similar to TON
Time delay period is from the instant the rung goes TRUE until the DN bit is turned on (like TON)

31. RTO can be used for...
Maintenance functions;
Diagnostic programs

32. Sheng-Jen (“Tony”) Hsieh, Texas A&M University
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).

33. Using the RTO Status Bits
Sheng-Jen (“Tony”) Hsieh, Texas A&M University
Using the RTO Status Bits
To reset the ACC value and status bits, you must program a reset (RES) instruction with the same address

34. Timer: RTO

35. 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

36. 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

37. RTO EN bit (like TON) Is set when rung conditions are TRUE
(like TON) Remains set until rung conditions go FALSE

38. RTO TT bit (like TON) Is set when (like TON) Remains set until
rung conditions are TRUE AND
AC < PR
(like TON) Remains set until
rung conditions go FALSE OR
when the DN bit is set

39. RTO DN bit (like TON) Is set when AC => PR
Remains set until the appropriate RES instruction is enabled

40. Close this window and
then select items 4, 5, 6
TON, TOF, and RTO
Simulation.