XIC vs XIO

This may be one of the most important lessons for people new to Ladder Logic. Often this is something people have a difficult time grasping. Especially people with physical electrical backgrounds.

XIC stands for Examine If Closed. And XIO stands for Examine If Open. And their ladder symbols may look similar to NO ( Normally Open ) and NC ( Normally Closed ) coil symbols on an electrical schematic. However they are very different.

NO and NC in physical electrical circuits are set in stone. Because it is a physical coil. However in Ladder Logic we use software coils. Software coils act in a monitoring function and depending on the situation can be pretty interchangeable.

• I will note that it is a best practice to always start a rung ( line of Ladder Logic ) with a XIC instruction. This is related to safety. This way you don’t energize something on startup, which could be dangerous.

Let me give some software coil examples to help you better understand this concept.

XIC – Examine if closed. If the circuit is closed, it will evaluate to true. And if the circuit is open, it will evaluate to false.

XIO – Examine if open. If the circuit is closed, it will evaluate to false. And if the circuit is open, it will evaluate to true.

Once you understand how these can work interchangeably, you have accomplished a great milestone.