Page 372 - Programmable Logic Controllers, Fifth Edition - Mobile version

Inputs Ladder logic program Outputs
L1 L2
Motor_Stop Motor_Start Track_Motor
<Local:1:I.Data.0> <Local:1:I.Data.1> <Local:2:O.Data.0> Do_Not_Enter_Sign
Motor_Stop

Track_Motor Enter_LS
Motor_Start
<Local:2:O.Data.0> <Local:1:I.Data.2> Wash_In_Use Track_Motor
L
Stop

Do_Not_Enter_Sign
Start Wash_In_Use <Local:2:O.Data.1>
Enter_LS

Exit_LS
<Local:1:I.Data.3> Wash_In_Use
U
Exit_LS

Figure 15-42 Latch/unlatch instruction used as part of a car wash program.

• When the OTL goes false, the output address will • When the car exits the car wash, the Exit_LS con-
remain a 1. tact momentarily closes and energizes the Wash_In_
• This is true even if the processor powers down and Use unlatch instruction.
then back up. • This in turn, de-energizes the Do_Not_Enter_Sign
• The output address will remain a 1 until reset to 0 to indicate the car wash is not in use.
by the unlatch instruction. • The track motor remains running ready to restart the
• If the output address is off, both the latch and un- process once another car enters.
latch instructions are not intensified, but once the bit • Once running, the track motor can be stopped at
is turned on, you will see both the latch and unlatch any time by operating the Motor_Stop button to de-
intensified even though both inputs are shut off. energize the Track_Motor.
Figure 15-42 shows an application of the latch/un-
latch instruction used as part of a car wash program. The One-Shot Instruction
car wash uses a conveyor chain that pulls cars along a The CLX One-Shot (ONS) instruction is an input instruc-
track. The operation of the program can be summarized tion used to turn an output on for one program scan only.
as follows: The program of Figure 15-43 uses the ONS instruction with
• The car wash only washes one car at a time. a math instruction to perform a calculation once per clos-
ing of the switch. This program is used to execute the ADD
• The operator controls the ON/OFF operation of math function only once per actuation of the limit switch,
the track motor with the track motor Stop and Start no matter how long the limit switch is held closed. The
pushbuttons. operation of the program can be summarized as follows:
• When the Motor_Start button is closed the Track_
Motor is energized and performs a seal-in function • On any scan for which limit_switch_1 is cleared or
that keeps the motor operating when the Motor_ storage_1 is set, this rung has no effect.
Start button is released. • On any scan for which limit_switch_1 is set and stor-
• As the car enters the car wash, the Enter_LS contact age_1 is cleared, the ONS instruction sets storage_1
momentarily closes to energize the Wash_In_Use and the ADD instruction increments sum by 1.
latch instruction. • As long as limit_switch_1 stays set, sum stays
• This in turn, energizes the Do_Not_Enter_Sign to the same value. The limit_switch_1 must go from
indicate that the car wash is in use. cleared to set again for sum to be incremented again.

Bit-Level Programming Part 2 353

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