Page 164 - Programmable Logic Controllers, Fifth Edition
P. 164

True
                                   Time input PB1
                                               False
                                      EN (enable) bit  On                                   Enable bit is reset
                                                 O                                          when input pushbutton
                                                           Accum = Preset                   PB1 is opened.
                                                                             8 9
                                                                           7
                                    Accumulated value                 5  6
                                    retained when rung              4
                                    condition goes false   2  3
                                                        1
                                                     0
                                  Accumulated value
                                                 On
                                      DN (done) bit
                                                 O
                                                 On
                                         PL output
                                                 O
                                                 On
                                    Reset input PB2
                                                 O
                                                     01   23 45 67 89        10 11 12
                                                                 Time in seconds

                                  Figure 7-28  Retentive on-delay timer timing chart.



                     •  When the accumulated value (9) equals the preset   •  If the timing rung goes false, the timer will stop
                       value (9), the timer done bit T4:2/DN is set to 1 and   timing but will recommence timing for the stored
                       the pilot light output PL is switched on.             accumulated value each time the rung goes true.
                     •  Whenever the momentary reset pushbutton is         •  When the reset PB2 is closed, the T4:2/DN bit is
                       closed, the timer accumulated value is reset to 0.    reset to 0 and turns the pilot light output off. The ac-
                                                                             cumulated value is also reset and held at zero until
                     Figure 7-28 shows a timing chart for the retentive on-
                   delay timer program. The timing operation can be sum-     the reset pushbutton is opened.
                   marized as follows:                                     The program drawn in Figure 7-29 illustrates a prac-
                                                                         tical application for an RTO. The purpose of the RTO
                     •  When the timing rung is true (PB1 closed), the
                       timer will commence timing.                       timer is to detect whenever a piping system has sustained




                                             Inputs             Ladder logic program         Output
                                       L1                                                            L2
                                                          S1                     T4:1
                                              Reset
                                                                                 RES
                                                   S1                                      Horn
                                          Key switch      PS
                                                                    RTO
                                                               RETENTIVE TIMER ON    EN
                                          Pressure             Timer             T4:1
                                           switch              Preset           60   DN
                                                   PS
                                                               Accumulated      0
                                                         T4:1                   Horn


                                                         DN
                                       Figure 7-29  Retentive on-delay timer alarm program.



                                                                                      Programming Timers  Chapter 7     145







          pet73842_ch07_131-155.indd   145                                                                              05/11/15   4:22 PM
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