Page 162 - Programmable Logic Controllers, Fifth Edition
P. 162

Input
                                     LS1       Ladder logic program                Outputs          Figure 7-24  Equivalent
                                                                                                    PLC program of the hardwired
                   L1                        TOF                                             L2     off-delay timer relay circuit
                                             TIMER OFF DELAY         EN                  OL
                                             Timer          T4:1                                    containing both instantaneous
                             LS1             Time base        1.0            M1                     and timed contacts.
                                             Preset            5     DN
                                             Accumulated       0                         OL
                                                                             M2
                                    T4:/DN                           M1
                                                                             G      G
                                    T4:/DN                           M2

                                                                             R      R
                                    T4:/EN                           G


                                    T4:/EN                           R




                     •  Instantaneous contact TD-3 closes to switch the    Figure 7-24 shows an equivalent PLC program of the
                       green light on, and instantaneous contact TD-4    hardwired off-delay timer relay circuit containing both
                       opens to switch the red light off.                instantaneous and timed contacts.  The timer instruction
                     •  After a 5-s time-delay period, timed contact TD-1   carries out all of the functions of the original physical timer.
                       closes to energize motor starter M1, and timed con-  Figure 7-25 shows a program that uses both the on-
                       tact TD-2 opens to de-energize motor starter M2.  delay and the off-delay timer instruction.  The process




                                                              PS1   PS2   PS3

                                                      Tank                          Tank
                                                       A                             B
                                                                   Pump

                                                               Ladder logic program

                                               Stop     Start       T4:5     PS1           Pump


                               Inputs                                                                   Output
                           L1                           Pump          DN                                         L2

                                      Stop    Pump                     TON  Pump delay                        OL
                                                                       TIMER ON DELAY       EN    Pump    M
                                      Start                            Timer          T4:6
                                                                       Time base        1.0  DN
                                                                       Preset            5
                                      PS1                              Accumulated       0
                                                  T4:6       PS2            Pump time
                                                                       TOF
                                                                       TIMER OFF DELAY      EN
                                      PS2            DN                Timer          T4:5
                                                             PS3       Time base        1.0  DN
                                                                       Preset           14
                                                                       Accumulated       0
                                      PS3


                           Figure 7-25  Fluid pumping process.



                                                                                      Programming Timers  Chapter 7     143







          pet73842_ch07_131-155.indd   143                                                                              05/11/15   4:22 PM
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