What is Examine if open (XIO) Instruction in PLC?

Use the Examine if open (XIO) instruction in your ladder program to determine if a bit is off.

  • When the XIO instruction is executed, if the bit addressed is on (1), then the instruction is evaluated as FALSE.

  • When the XIO instruction is executed, if the bit addressed is off (0), then the instruction is evaluated as TRUE.

Alternate name: Normally closed contact.

Examine if open (XIO) instruction

An XIO instruction inverts the state of the bit it references.


The “Examine if Open” (XIO) instruction is a commonly used instruction in programmable logic controllers (PLCs). It is typically used to check the status of an input (bit) and determine if it is not energized or turned off.

The XIO instruction is a logical instruction and is represented by a symbol such as a contact or a coil with a diagonal line through it. It is often referred to as a “normally closed” (NC) contact in ladder logic programming.

XIO instruction

Input Status Check: The XIO instruction examines the status of a specific input or bit. It checks whether the input is in an “off” or “false” state.

Output Condition: If the input being examined by the XIO instruction is not energized (i.e., it is off), the XIO instruction allows the logic to flow to the output connected to it.

Output Activation: When the XIO instruction determines that the input is not energized (off), it activates the output connected to it. The output can be used to control other devices or trigger specific actions within the PLC program.

In ladder logic programming, the XIO instruction is commonly used in conjunction with other instructions such as “Examine if Closed” (XIC), “Output Energize” (OTE), and timers/counters to create complex control sequences and logic.

The XIO instruction allows the programmer to conditionally activate outputs based on the status of inputs, providing flexibility and control in PLC programming.