The examine if on (XIO), is used to examine the reference address for an on condition
to provide continuity through the contact.
For example, if a push button is connected to an input module, and the address of connection
is 10, and you program an exam if on (XIO) instruction to represent PB1.
The processor when reading the status of the field device, will read a zero if it is not
pushed because there is no voltage.
A normally open contact will remain open.
If the pushbutton is pressed, then the processor will read a 1 and the normally open contact
will close thus providing continuity in the PLC ladder rung.
As soon as the push button is released, the contact will go back to open.
In a PLC, you will also see the contacts highlighted when there is power at the input module with
that address.
If the contact is not highlighted it means there is no power continuity and it’s reference
device is off.
For more PLC training, go to http://www.bin95.com and click on PLC training videos on the left
hand side.
You can learn more about how a PLC works and PLC automation basics.