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programmable logic controllers or plc's
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are the backbone of Industrial
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Automation they control machines and
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processes in factories worldwide today
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we're focusing on two fundamental
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concepts in TLC logic normally open and
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normally closed contacts normally open
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or n and normally closed or NC refer to
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the default state of electrical contacts
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in a circuit a normally open contact is
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open by default meaning it doesn't allow
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current to flow until activated on the
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other hand a normally closed contact is
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closed by default allowing current to
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flow until it's activated in PLC logic
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these Concepts translate to how inputs
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are interpreted a normally open input is
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considered off or false until it's
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activated while a normally closed input
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is considered on or true until it's
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activated this distinction is crucial
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for proper PLC programming and operation
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when it comes to wiring normally open
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devices are connected in parallel to the
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input when any of these devices activate
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they complete the circuit sending a
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signal to the PLC normally closed
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devices are wired in series when all
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devices are in their normal State the
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circuit is complete if any device
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activates it breaks the circuit
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PLC programming for no and NC inputs in
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a PLC involves using the appropriate
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instruction for each type for a normally
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open input you would use a normal
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examine instruction often represented by
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a pair of vertical lines for a normally
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closed input you'd use an examine closed
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instruction typically shown as a pair of
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vertical lines with a diagonal line
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through them let's consider a practical
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example imagine a safety system for a
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machine emergency stop buttons are
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typically wired as normally closed when
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the button is not pressed the circuit is
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complete allow ing the machine to
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operate if the button is pressed it
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opens the circuit immediately stopping
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the machine on the other hand a start
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button would be normally open it only
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completes the circuit when pressed
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signaling the PLC to start the
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machine understanding the difference
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between normally open and normally
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closed logic is essential for Designing
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safe and efficient control systems by
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choosing the appropriate type for each
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application you can create more reliable
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and intuitive PLC programs
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remember the key is to consider the
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default state of your inputs and how you
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want your system to behave in both
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normal operation and fault conditions to
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learn more about PLC programming click
