Page 160 - Programmable Logic Controllers, Fifth Edition
P. 160

Input
                                                         I:1/0  Ladder logic program           Output
                                      L1                       TOF                                     L2
                                                               TIMER OFF DELAY       EN
                                           S1                  Timer         T4:3
                                                 I:1/0         Time base      1.0   DN      O:2/1  PL
                                                               Preset          15
                                                               Accumulated      0
                                                       T4:3/DN                      O:2/1




                                                          True
                                               S1 input
                                            enable bit (EN)         False


                                             Timed period timing bit (TT)
                                                                           15 s
                                                                          O  delay
                                                                        timed duration
                                                       True (logic 1)
                                            Timed output
                                            done bit (DN)                                False (logic 0)
                                                O:2/1
                                                       Preset value = accumulated value
                                      Figure 7-21  Off-delay programmed timer.






                   the instruction goes from ON to OFF or from true to       to 1, immediately turning on motors M1, M2,
                   false. The operation of the circuit can be summarized     and M3.
                   as follows:                                             •  If SW is then opened, logic continuity to all three

                     •  When the switch connected to input I:1/0 is first    timers is lost and each timer begins counting.
                       closed, timed output O:2/1 is set to 1 immediately   •  Timer T4:1 times out after 5 s resetting its done bit
                       and the lamp is switched on.                          to zero to de-energize motor M1.
                     •  If this switch is now opened, logic continuity is lost
                       and the timer begins counting.
                     •  After 15 s, when the accumulated time equals the                 Timer OFF DELAY (TOF)
                       preset time, the output is reset to 0 and the lamp                           Enable Bit (EN)   1
                       switches off.                                              TOF            Timer Timing Bit (TT)  0
                                                                                Instruction
                     •  If logic continuity is gained before the timer is timed    ON               Done Bit (DN)     1
                                                                                                    Accumulating
                       out, the accumulated time is reset to 0. For this rea-                       Enable Bit (EN)  NO
                                                                                                                      0
                       son, this timer is also classified as nonretentive.        TOF            Timer Timing Bit (TT)  1
                                                                                Instruction         Done Bit (DN)     1
                     Figure  7-22 illustrates the use of an off-delay timer       OFF               Accumulating     YES
                   instruction used to switch motors  off sequentially at                           Enable Bit (EN)   0
                   5 second intervals. The operation of the program can be      Timed Out        Timer Timing Bit (TT)  0
                   summarized as follows:                                     Accum = Preset        Done Bit (DN)     0
                                                                                                    Accumulating     NO
                     •  Timer preset values for T4:1, T4:2, and T4:3 are set                        Enable Bit (EN)   1
                       for 5, 10, and 15 s, respectively.                     Instruction OFF    Timer Timing Bit (TT)  0 1
                                                                                                    Done Bit (DN)
                                                                               after timed out
                     •  Closing the input switch SW immediately sets                                Accumulating    Reset
                       the done bit of each of the three off-delay timers   Table showing how each bit is e ected during the program operation.





                                                                                      Programming Timers  Chapter 7     141







          pet73842_ch07_131-155.indd   141                                                                              05/11/15   4:21 PM
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