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Jack H.Integration and automation of manufacturing systems.2001.pdf
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page 146

7.3.2 Ladder Logic Inputs

Contact coils are used to connect the PLC power lines to drive the outputs.

The inputs can come from electrical inputs or memory locations.

Normally open, an active input will close the contact and allow power to flow.

Normally closed, power flows when the input is not active.

Note: if we are using normally closed contacts in our ladder logic, this is independent of what the actual device is. The choice between normally open or closed is based on what is logically needed, and not the physical device.

For the Micrologix PLCs the inputs are labelled ‘I:0.0/x’ where x is the input number 0 to 9.