Following table shows the PLC Programming Ladder Logic Instructions.
1. Contact (NO)
Contact instruction maintains its state same as present state of the Tag which is linked with it.
Refer below diagram which shows Contact instruction with Tagname, Point “A” & Point “B”. Point “A” is the input side of the instruction and Point “B” is the Output side of the instruction.
If the value of Tagname is Logic Low or “0”, the Contact instruction becomes deactivated. At this time result of the instruction (point “B”) is Logic Low or “0”, irrespective of the logic value at point “A”. Refer below diagram.
If value of Tagname is Logic High or “1”, the Contact instruction becomes activated. At this time result of the instruction (point “B”) is same as logic value at point “A”. Refer the below diagram.
2. Inverted Contact (NC)
Inverted Contact instruction maintains its state as opposite to the present state of the Tag which is linked with it.
If the value of Tagname is Logic Low or “0”, the Contact instruction becomes activated. At this time result of the instruction ( at point “B”) is the same as the logic value at point “A”. Refer below diagram.
If the value of Tagname is Logic High or “1”, the Contact instruction becomes deactivated. At this time result of the instruction (point “B”) is Logic Low or “0”, irrespective of the logic value at point “A”. Refer below diagram.
3. Coil or Output Coil
Coil or Output coil instruction writes or sends its value to the Tag, which is linked with it.
Refer below diagram which shows Coil instruction with Tagname, and Point “A”. Point “A” is the input side of the instruction which controls the status of Tag via coil instruction.
The Coil instruction state is the same as the logic value at point “A”. Refer below diagram. And the same state moves to the Tagname by the coil instruction.
Note: First 2 instructions (Contact & Inverted Contact) gets the values from the Tag so it is called Input Instructions. And 3rd Instruction – coil gives the values to the Tag so it is called Output Instruction.
Ladder Logic Examples:
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