The XIC (Examine if Closed), a normally open switch or pushbutton, instruction is a fundamental instruction for working with Programmable Logic Controllers (PLCs). The XIC instruction performs the function opposite to the XIO (Examine if Open) instruction. When the input is closed, the output of the XIC will be ON. If the input is open, then the output of the XIC will be OFF.

The main purpose of using this type of instructions is to check status or condition of an equipment before allowing PLC to take any further action. For example, in a production line where machines are triggered by sensors to start operation, it is important to make sure that all safety doors are closed before starting production. In this case, inputs from door sensors are connected to PLC and programmed with XIC instructions. Only when all doors are confirmed closed, then PLC can allow subsequent actions such as machine start-up sequence to take place.

In summary, Examine if Closed (XIC) instruction in a Programmable Logic Controller performs an important function of checking whether an input device is turned off or not before taking any further action.

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