Page 384 - Programmable Logic Controllers, Fifth Edition - Mobile version

Tag Name Value Style Data Type timed event-driven routines. In this program the timers
-LS_Timer {…} TIMER are cascaded in such a manner that one event leads to an-
-LS_Timer.PRE 120000 Decimal DINT other. The operation of the program can be summarized
-LS_Timer.ACC 0 Decimal DINT as follows:
LS_Timer.EN 0 Decimal BOOL
LS_Timer.TT 0 Decimal BOOL • The process consists of three distinct timed events
LS_Timer.DN 0 Decimal BOOL or steps.
Limit_Switch 0 Decimal BOOL • Each step must last for a predetermined period of
LS_EN_PL 0 Decimal BOOL time and must be performed in a specific order.
LS_TT_PL 0 Decimal BOOL • The Start_PB, associated internal relay seal-in circuit,
LS_Alarm 0 Decimal BOOL
is activated to start the sequence.
Figure 15-63 Tags created for the RTO retentive on-delay • Event_1 has a 3 second duration, after which
timer program. Event_2 begins.

• Event_2 has a 10 second duration, after which
• When the Limit_Switch is closed and stays closed Event _3 begins.
until the timer times out, the status and value of the • Event_3 has a 5 second duration, after which the
instructions would be: entire process is completed.
- PRE – 120000
- ACC –120000 • The output pilot lights turn on to signify the com-
- LS_Timer.EN – 1 pletion of each step in the sequence.
- LS_Timer.TT – 0 • Once the final step has been completed, the
- LS_Timer.DN – 1 Process_Complete_PL is turned on.
- LS_EN_PL – 1 • Activating the Stop_PB at any time will reset the
- LS_TT_PL – 0 process.
- LS_Alarm – 1 Reciprocating timers are timing functions where the out-
• When the Limit_Switch is opened after the timer put of one timer is used to reset the input of a second timer,
times out, the status and value of the instructions each resetting the other. These types of timers are used in
would be: situations where a constant cycling of an output is required.
- PRE – 120000 For example, if a flashing light is required in the event of a
- ACC –120000 control system failure, a program with reciprocating timers
- LS_Timer.EN – 0 could be used to create the flashing output function, as
- LS_Timer.TT – 0 illustrated in the PAC program of Figure 15-65. The opera-
- LS_Timer.DN – 1 tion of the program can be summarized as follows:
- LS_EN_PL – 0
- LS_TT_PL – 0 • When the contacts of the Limit_Switch close,
- LS_Alarm – 1 Timer_Reciprocationg_Off beings its timing cycle.
• When the Reset_LS_Timer is closed, the status and • After two seconds, the Timer_Reciprocationg_Off.
value of the instructions are reset to their original DN bit status changes to true and both the Timer_
values. Reciprocating_On instruction and the flashing
Alarm_PL become energized.
Cascading of Timers • After two more seconds, the Timer_Reciprocating_

Timers can be linked together or cascaded to extend On.DN bit status changes to reset the sequence.
their control capability. The industrial control program of
Figure 15-64 is an example of cascading TON timers for

Programming Timers Part 3 365

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