Page 187 - Programmable Logic Controllers, Fifth Edition - Mobile version

Enter Ladder logic program
L1 Inputs Output L2
switch
CTU
Enter COUNT-UP COUNTER CU Lot full
switch Counter C5:1 DN light
Preset 150
Accumulated 0
Exit
switch
CTD
Exit COUNT-DOWN COUNTER CD
switch Counter C5:1
Preset 150 DN
Accumulated 0 Lot full
C5:1/DN light

Reset
Reset
C5:1
RES

Figure 8-23 Parking garage counter.

• As a car leaves, the exit switch triggers the down- Input Ladder logic program Output
counter output instruction and decrements the accu- L1 CTD L2
mulated count by 1. I:1/0 I:1/0 COUNT-DOWN COUNTER 0:2/0
Counter C5:0 CU
• Because both the up- and down-counters have the Preset 2 PL
same address, C5:1, the accumulated value will Accumulated 4 DN
be the same in both instructions as well as the
preset.
C5:0
• Whenever the accumulated value of 150 equals 0:2/0
the preset value of 150, the counter output is DN
energized by the done bit to light up the Lot
Full sign. Figure 8-24 Count-down counter instruction.
• A reset button has been provided to reset the accu-
mulated count.
• When the input rung condition makes another
Figure 8-24 shows an example of the count-down false-to-true transition, the accumulated value will
counter instruction used as part of the Allen-Bradley decrease to 2.
SLC 500 controller instruction set. The information to be
entered into the instruction is the same as for the count-up • When the input makes one more false-to true transi-
counter instruction. tion, the accumulated value will drop to 1.
The CTD instruction decrements its accumulated value • At this point the accumulated value of 1 is less than
by 1 every time it is transitioned. It sets its done bit when the preset value of 2 so the done bit will be reset (0)
the accumulated value is equal to or greater than the preset de-energizing output O:2/0.
value. The program of Figure 8-24 contains a count-down
counter instruction, the operation of which can be sum- Figure 8-25 shows an up/down-counter program
marized as follows: that will increase the counter’s accumulated value
when pushbutton PB1 is pressed and will decrease the
• With the program in the state shown, the CTD done counter’s accumulated value when pushbutton PB2
bit will be set (1) and output 0:2/0 will be energized is pressed. Note that the same address is given to the
because the accumulated value of 4 is greater than up-counter instruction, the down-counter instruction,
the preset value of 2. and the reset instruction. All three instructions will be
• When the CTD instruction rung makes a looking at the same address in the counter file. When
false-to-true transition, the accumulated value input A goes from false to true, one count is added to
decreases by one count to 3. the accumulated value. When input B goes from false

168 Chapter 8 Programming Counters

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