PLC TIMER FUNCTIONS

• From the LogixPro Simulations Menu, select the I/O Simulation.
• Clear out any existing program by selecting the "New" entry in the File menu, and then select the "Clear Data Table" entry in the Simulations menu.
• Now enter the following program being careful to enter the addresses exactly as shown.
• Confirm that you have entered the number 100 as the timer's preset value. This value represents a 10 second timing interval (10x0.1) as the timebase is fixed at 0.1 seconds:
•  Once you have your program entered, and have ensured that it is correct, download it to the PLC.

• Ensure that Switch I:1/0 is Open, and then place the PLC into the Run mode.
• Right click on the Timer instruction, and select "GoTo DataTable" from the drop-down menu.
• Note the initial value of timer T4:1's accumulator and preset in the spaces below.
•  Also indicate the state of each of the timer's control bits in the spaces provided:

     Initial State (Switch I:1/0=Open):

•     T4:1.ACC = _____ T4:1.PRE = ______ T4:1/EN = ____ T4:1/TT = ____ T4:1/DN = ____        Close switch I:1/0, and carefully observe the incrementing of the timer's accumulator, and         the state   of  each of it's control bits.
•     Once the Timer stops incrementing, note the final value of timer T4:1's accumulator, preset,       and the state of it's control bits below:

     Final State (Switch I:1/0=Closed):

•    T4:1.ACC = _____ T4:1.PRE = ______ T4:1/EN = ____ T4:1/TT = ____ T4:1/DN = ____ .
•    Toggle the state of switch I:1/0 a number of times, and observe the operation of the timer in      both the DataTable display and in the Ladder Rung program display.
•    Confirm that when the rung is taken false, the accumulator and all 3 control bits are reset to      zero.       
•  This type of timer is a non-retentive instruction, in that the truth of the rung can cause the      accumulator and control bits to be reset (=0).

Conclusions:

Use the TON instruction to turn an output on or off after the timer has been on for a preset time interval. This output instruction begins timing when its rung goes "true". It waits the specified amount of time (as set in the PREset), keeps track of the accumulated intervals which have occurred (ACCumulator), and sets the DN (done) bit when the ACC (accumulated) time equals the PRESET time.

As long as rung conditions remain true, the timer adjusts its accumulated value (ACC) each evaluation until it reaches the preset value (PRE). The accumulated value is reset when rung conditions go false, regardless of whether the timer has timed out.