timed event-driven routines. In this program the timers are cascaded in such a manner that one event leads to another. The operation of the program can be summarized as follows: • The process consists of three distinct timed events or steps. • Each step must last for a predetermined period of time and must be performed in a specific order. • The Start_PB, associated internal relay seal-in circuit, is activated to start the sequence. • Event_1 has a 3 second duration, after which Event_2 begins. • Event_2 has a 10 second duration, after which Event _3 begins. • Event_3 has a 5 second duration, after which the entire process is completed. • The output pilot lights turn on to signify the completion of each step in the sequence. • Once the final step has been completed, the Process_Complete_PL is turned on. • Activating the Stop_PB at any time will reset the process. Reciprocating timers are timing functions where the output of one timer is used to reset the input of a second timer, each resetting the other. These types of timers are used in situations where a constant cycling of an output is required. For example, if a flashing light is required in the event of a control system failure, a program with reciprocating timers could be used to create the flashing output function, as illustrated in the PAC program of Figure 15-65. The operation of the program can be summarized as follows: • When the contacts of the Limit_Switch close, Timer_Reciprocationg_Off beings its timing cycle. • After two seconds, the Timer_Reciprocationg_Off. DN bit status changes to true and both the Timer_ Reciprocating_On instruction and the flashing Alarm_PL become energized. • After two more seconds, the Timer_Reciprocating_ On.DN bit status changes to reset the sequence. Tag Name Value Style Data Type {…} 120000 0 0 0 0 TIMER DINT DINT BOOL BOOL BOOL BOOL BOOL -LS_Timer -LS_Timer.PRE -LS_Timer.ACC LS_Timer.EN LS_Timer.TT LS_Timer.DN Limit_Switch LS_EN_PL 0 0 0 0 Decimal Decimal Decimal Decimal Decimal Decimal Decimal Decimal Decimal BOOL BOOL LS_TT_PL LS_Alarm Figure 15-63 Tags created for the RTO retentive on-delay timer program. • When the Limit_Switch is closed and stays closed until the timer times out, the status and value of the instructions would be: -- PRE – 120000 -- ACC –120000 -- LS_Timer.EN – 1 -- LS_Timer.TT – 0 -- LS_Timer.DN – 1 -- LS_EN_PL – 1 -- LS_TT_PL – 0 -- LS_Alarm – 1 • When the Limit_Switch is opened after the timer times out, the status and value of the instructions would be: -- PRE – 120000 -- ACC –120000 -- LS_Timer.EN – 0 -- LS_Timer.TT – 0 -- LS_Timer.DN – 1 -- LS_EN_PL – 0 -- LS_TT_PL – 0 -- LS_Alarm – 1 • When the Reset_LS_Timer is closed, the status and value of the instructions are reset to their original values. Cascading of Timers Timers can be linked together or cascaded to extend their control capability. The industrial control program of Figure 15-64 is an example of cascading TON timers for Programming Timers Part 3 365
Petruzella_ProgrammableLogicControllers__5e
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