LogixPro
Introduction To
RSLogix Timers

The TON Timer .... (Timer ON Delay)

• 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.

Cascaded TON Timers

• Insert a new rung containing a second timer just below the first rung as shown below. This second timer T4:2 will be enabled when the first timer's Done bit T4:1/DN goes true or high (1).

• Once you have completed this addition to your program, download your program to the PLC and select RUN.
• Toggle the state of switch I:1/0 to ON and observe the operation of the timers in your program.
• Bring the DataTable display into view, and pay particular attention to the way in which the timers are cascaded (one timer starts the next).
• Try changing the value of one of the timer presets by double clicking on the preset value in the DataTable display, and then entering a new value.
• Run the timers through their timing sequence a number of times. Don't move on until you are satisfied that the timers are working as you would expect

In this exercise we have utilized just two timers, but there is nothing stopping us from sequencing as many timers as we wish. The only thing to remember is; to use the DN (done) bit of the previous timer to enable the next timer in the sequence. Obviously locating the timers on consecutive rungs, and employing consecutive numbering will make such a program much easier to read and trouble-shoot.

Self Resetting Timers

• Place the PLC into the PGM mode, and modify the first rung of your program as depicted below.

• Once you have modified your program, download it to the PLC and place the PLC into the RUN mode.
• Close switch I:1/0 and observe the operation of the timers. The timers should now be operating in a continuous loop with Timer1 starting Timer2, and then when Timer2 is done, Timer1 is reset by Timer2's done bit. As before, when Timer1 is reset, it in turn resets Timer2 which causes Timer2's done to go low (T4:2/DN=0). Once Timer2's done bit is low, the sequence is back to where it originally began, and the timing sequence will start over once again on the very next scan.
• Remove the first instruction (switch XIC I:1/0) from rung zero of your program.
• Download and RUN this modified version of your program
• Does the timing operation continuously sequence as before? It should!
• Can you stop the timing sequence? Not without taking the PLC out of the RUN mode! In many applications there may never be a need to stop such a timing sequence, so a switch might not be used or needed.

In this exercise we cascaded two timers, but as before there is nothing to stop us from cascading as many timers as we wish. The thing to remember here is; utilize the DN (XIC or "NOT"done) bit of the last timer in the sequence to reset the first timer in the sequence. Once again, consecutive rungs, and numbering will make a program much easier to read and trouble-shoot.

The TOF Timer .... (Timer OFF Delay)

In Allen Bradley PLC programming, the TON timer is by far the most commonly used type of timer. Most people consider TON timers to be simple to use and understand. In comparison, many people find the operation of the Allen Bradley TOF (Timer OFF delay) timer to be less intuitive, but I'm going to let you decide for yourself.

• Make sure that switch I:1/0 is Closed, and then enter or modify your existing program to match the one shown below.