A Direct Contact is a normally open (NO) contact condition.
A contact is always linked to a bit. It represents an action or condition.
Each contact condition in a net is evaluated to determine the coil (output or expression) condition. Contacts can be connected in series and in parallel on a Ladder net.
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A Direct Contact is a normally open (NO) contact condition. |
A door buzzer is an example of a Direct Contact. When you push the buzzer, power flows through the circuit and the buzzer sounds. When you release the buzzer, the sound stops.
During the system scan, the processor evaluates the program elements net by net.
If the Direct Contact bit operand (the door buzzer) is OFF (logic 0): power will not flow through the Direct Contact. The door buzzer is silent.
If the Direct Contact address (the door buzzer) is ON (logic 1): power will flow through the Direct Contact. The door buzzer sounds.
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An Inverted Contact represents a normally closed contact condition. |
An Inverted Contact condition can be from an external input device (for example: a push button) or from an internal input system element (for example: SB 50 Key +/- is pressed).
An emergency light contains an example of an Inverted Contact.
Normally,
there is power flow through the emergency light's Inverted Coil and the
light stays off.
During
an electric power outage, the power flow through the Inverted Coil stops
and the emergency light comes on.
During the system scan, the processor evaluates the program elements net by net.
If the Inverted Contact address (power supply) is ON (logic 1): power will not flow through the Inverted Contact. The emergency light will stay off.
If the Inverted Contact address (power supply) is OFF (logic 0): power will flow through the Inverted Contact. The emergency light turns on.
If the power outage ends and power flow is returned to the Inverted Contact, it will close and the emergency light will again turn off
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A Positive Transition Contact gives a single one-shot pulse when the bit operand it is linked to rises from OFF (logic 0) to ON (logic 1). |
A Negative Transition Contact gives a single one-shot pulse when the bit operand it is linked to falls from ON (logic 1) to OFF (logic 0).
A cellular phone keypad key is an example of a Positive Transition Contact. When you push a key a number is displayed on the screen. It does not matter if you push the key quickly or hold it down for several seconds. The number will only appear once on the screen.
The cellular phone registers the transition from key NOT pressed to key pressed. The length of time the key is pressed is not relevant. You must release the key and press it again to repeat the number on the cellular phone screen.
During the system scan, a Positive Transition Contact address is evaluated for a transition from OFF to ON. A transition allows power to flow through the Positive Transition Contact for one scan.
At the end of a scan, the Positive Transition Contact is reset to ON (logic 1). The Positive Transition Contact is re-activated when the linked signal turns from OFF to ON.
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A Negative Transition Contact gives a single one-shot pulse when the bit operand it is linked to falls from ON (logic 1) to OFF (logic 0). |
A computer ON/OFF button is an example of a Negative Transition Contact. The computer is ON.
If you push the ON/OFF button in without releasing it, the computer will not shut down. But when you release the button, the system registers a change in status from ON to OFF. The computer then shuts down.
During the system scan, a Negative Transition Contact address is evaluated for a transition from ON to OFF. A transition allows power to flow through the Negative Transition Contact for one scan.
At the end of a scan, the Negative Transition Contact is reset to OFF (logic 0). The Negative Transition Contact can only be re-activated when the triggering signal again changes from ON to Off.
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The NOT contact functions like a type of logic gate sometimes called an inverter. |
If the RLO that reaches the NOT contact is 1, the contact outputs an RLO of 0.
If the RLO that reaches the NOT contact is 0, the contact outputs an RLO of 1.
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