1. Ladder Logic
PLC Programs are made up of combinations of AND; OR; NAND; NOR; and other gates, along with timers, inputs, outputs, counters, comparators, and other components. These gates are programmed in a method called Ladder Logic.
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2. Ladder Logic
To control the flow of electricity, contacts must be opened and closed. Contacts are represented in ladder logic in this manner.
Open Closed
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3. Ladder Logic
PLCs only care about which output is turned on and not what’s physically connected to the output.
Hot line
Neutral line
Contact (input) Coil (output)
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4. Ladder Logic
Instead of open or closed, gates are considered true or false, based on their logic chart output. In this example, column C.
A B C
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5. Ladder Logic
Since this is an AND gate, the output condition would be true only if both contacts A and B were true. The output Q would then be true.
A B C
(Q)
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6. Ladder Logic
Since this is an OR gate, the output condition would be true if either contacts A and B were true. The output Q would then be true.
A B C
(Q)
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7. Ladder Logic
Since this is an NOR gate, the output condition would be true only if both contacts A and B were false. The output Q would then be true.
A B C
(Q)
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8. Ladder Logic
Since this is a NAND gate, the output condition would be true if either contacts A and B were false. The output Q would then be true.
A B C
(Q)
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9. Ladder Logic
Counters exist in the PLCs memory. They are software based and can count up or down. CU CD S
CW DU DE
(Q)
R Q
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10. Ladder Logic
Timers are like counters in that they exist in the PLCs memory. They are software based and can be on or off delays, or pulse based. On delay timers are the most common.
T! - !0
TW DU DE
(Q)
R Q
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11. Ladder Logic
At the bottom of the ladder, a symbol designates the end of the program.
END
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12. Here is a more complex ladder. Can you figure it out
Here is a more complex ladder. Can you figure it out? Let’s look at it in sequence.
Ladder Logic
(Q)
(R)
END
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13. Ladder Logic
Say you have a cistern in your basement to store water being pumped from a well. You want the well pump to supply water when the reservoir is low, but stop pumping when it is full. If your well pump runs constantly, it will burn out.
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14. Two sensors (inputs 1 & 2) will control the output (Q) /3.
Ladder Logic
(Q)
False
False 1 2 3
(R) 3
END
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15. One relay (input 3) will run the pump ®. This is also output (Q).
Ladder Logic
(Q)
False
False 3
(R)
False 3
END
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16. Normally closed sensors (inputs 1 & 2) will be open as the tank is empty to begin with. This will cause a TRUE signal to be sent out 3
to the output (Q)
This output is
also the input
(3) relay to the
motor ®.
Ladder Logic
END
(Q)
(R) 1 3 2
True
True
True
True
True
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17. As the tank fills, the “low” sensor (1) will close, but the relay signal (3) remains TRUE. There is a TRUE connection from (3) to
(2). Therefore,
the pump
continues to fill
the tank.
Ladder Logic
END
(Q)
(R) 1 3 2
True
False
True
True
True
True
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18. Ladder Logic (Q) (R) When the “full” sensor
(2) is also closed, the signal to relay 3 becomes FALSE, and the motor stops.
Ladder Logic
END
(Q)
(R) 1 3 2
False
False
True
False
True
False
True
False
True
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19. As water is used, the full sensor will open, and turn to a state of TRUE, but there is no signal path until the low sensor is also tripped
to TRUE.
Ladder Logic
END
(Q)
(R) 1 3 2
False
False
True
False
False
False
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20. Once both sensors are TRUE again, the cycle can begin again, repeating as necessary to refill the tank as water is used.
Ladder Logic
END
(Q)
(R) 1 3 2
False
True
True
False
True
False
True
True
False
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21. Ladder Logic
PLCs can control as few as a couple of input outputs, to as many as 40,000 or more.
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