What instruction is used to check if a condition is open?

The instruction used to check if a condition is open is the XIO instruction. XIO stands for "eXamine Input Open" and it is designed to test the state of a particular input condition within a programmable logic controller (PLC). When this instruction is executed, it evaluates the specified condition; if the condition is not true (or "open"), it will allow the program to continue its execution.

This type of conditional check is particularly useful in control logic, as it helps determine the flow of operations based on sensor states or control signals. For instance, if you want to ensure that an input does not signal an active condition, a XIO instruction would be the appropriate choice.

The other options serve different purposes in control logic: for example, the XIC instruction checks if a condition is closed, the OTL instruction is used to turn on a bit, and the OTE instruction is used to turn on an output bit. Understanding the specific functions of these instructions is critical for effectively programming and troubleshooting within a PLC environment.