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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. ｩEmil Decker, 2009

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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 ｩEmil Decker, 2009

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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) ｩEmil Decker, 2009

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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 ｩEmil Decker, 2009

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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) ｩEmil Decker, 2009

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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) ｩEmil Decker, 2009

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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) ｩEmil Decker, 2009

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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) ｩEmil Decker, 2009

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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 ｩEmil Decker, 2009

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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 ｩEmil Decker, 2009

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Ladder Logic At the bottom of the ladder, a symbol designates the end of the program. END ｩEmil Decker, 2009

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**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 ｩEmil Decker, 2009

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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. ｩEmil Decker, 2009

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**Two sensors (inputs 1 & 2) will control the output (Q) /3.**

Ladder Logic (Q) False False 1 2 3 (R) 3 END ｩEmil Decker, 2009

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**One relay (input 3) will run the pump ®. This is also output (Q).**

Ladder Logic (Q) False False 3 (R) False 3 END ｩEmil Decker, 2009

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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 ｩEmil Decker, 2009

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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 ｩEmil Decker, 2009

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**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 ｩEmil Decker, 2009

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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 ｩEmil Decker, 2009

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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 ｩEmil Decker, 2009

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Ladder Logic PLCs can control as few as a couple of input outputs, to as many as 40,000 or more. ｩEmil Decker, 2009

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