PLC EXERCISES · DRILL 1 OF 3
XIC and XIO explainedTurn on a light with ladder logic.
Tap each slot to place an instruction, then press the push button and watch your rung run. Five quick steps.
Ladder logic reads like an electrical diagram: power flows from the left rail, through contacts, to an output on the right. Each contact looks at one bit. XIC ] [ passes power while its bit is ON. XIO ]/[ passes power while its bit is OFF. OTE ( ) turns its bit ON while the rung is true.
Build each rung below, then try it with the push buttons. The PLC runs your rungs live, even wrong ones, so you can see what happens.
XIC, XIO and OTE quick reference
| Instruction | Symbol | True when |
|---|---|---|
| XIC: examine if closed | ] [ | Its bit is ON (1) |
| XIO: examine if open | ]/[ | Its bit is OFF (0) |
| OTE: output energize | ( ) | Turns its bit ON while the rung is true, OFF while it’s false |
Series and parallel
- Contacts in series must all be true: that’s AND.
- Contacts in a branch (parallel) need only one true: that’s OR.
The common mix-up
XIO doesn’t mean “normally closed button.” It means “true while this bit is OFF.” What the bit is depends on how the device is wired. A normally closed Stop button’s bit is ON until it’s pressed, so it’s usually examined with an XIC. The seal-in circuit drill uses exactly that.
Common questions
What is the difference between XIC and XIO?
XIC (examine if closed) is true while its bit is ON. XIO (examine if open) is true while its bit is OFF. Both look at the same kind of bit; they just ask opposite questions.
Is XIO the same as a normally closed contact?
It acts like a normally closed contact on the bit, but it says nothing about how the field device is wired. A normally closed Stop button’s input bit is ON until it’s pressed, so it’s usually examined with an XIC.
What does OTE do?
OTE (output energize) turns its bit ON while the rung is true and OFF while it’s false, every scan.
How do you make AND and OR in ladder logic?
Contacts in series are AND: all must be true. Contacts in a parallel branch are OR: any one can be true.
YOUR NEXT STEP
See a real program
Level 1’s PLC program runs a motor with the same instructions: XIC, XIO, OTE and a seal-in branch. It’s free.
Training practice. Instruction names follow the common IEC/Allen-Bradley style (XIC, XIO, OTE); other PLC brands use different names for the same contacts and coils.