Contacts (input instruction)
XIO: Examine If Open
Passes power when the bit it is examining is 0.
Also called: normally closed contact, NC contact, examine off, -|/|-.
XIO(tag)What it does
XIO is XIC inverted. It passes power while the bit is 0 and blocks it while the bit is 1. Everything else about it, series is AND, parallel is OR, is identical.
Its most important job is the stop button, and understanding why is worth more than memorising the symbol. A stop button is wired normally closed: the circuit is complete while nobody is pressing it, so the input bit reads 1 during normal running. Press the button and the circuit opens, the bit drops to 0, and an XIO on that bit starts passing power. That looks backwards until you notice what it buys you: if the wire to that button is ever cut, the bit also goes to 0, and the machine stops exactly as if someone had pressed stop. The failure mode is safe by construction.
The same reasoning applies to any permissive that must fail safe: an overload contact, a guard switch, an emergency stop chain.
A rung you can run
A stop that also fails safe on a broken wire
[XIC(Start_PB),XIC(Motor_Run)]XIO(Stop_PB)XIO(Overload_Trip)OTE(Motor_Run)Both Stop_PB and Overload_Trip are wired normally closed, so both bits are 1 while the plant is healthy and the XIO instructions pass power. Cut either wire and the motor drops out.
Run this rung in the simulatorWhat it is called on other controllers
| Platform | Equivalent |
|---|---|
| Rockwell (Studio 5000) | XIO |
| Siemens (TIA Portal) | Normally closed contact, drawn -|/|- |
| Mitsubishi (GX Works) | LDI / ANI / ORI |
| CODESYS and IEC 61131-3 | Negated contact |
Common mistakes
- Wiring the stop button normally open and using XIO anyway, which produces a machine that stops when everything is fine and runs when the wire breaks. Get the wiring and the instruction consistent, and make the safe state the de-energised one.
- Using XIO on a fault bit and expecting the rung to recover on its own. If the fault is latched with OTL, XIO will keep blocking until something unlatches it.
Practise it
Interactive exercises that use XIO. Each one checks your ladder against a real scan.
- Start / Stop CircuitThe first circuit every controls engineer learns: a motor that starts on a button press, keeps running when the button is released, and stops on a stop button, with stop always winning.
- State Coil / Fault CoilA fault lamp that latches ON when any alarm condition occurs, even briefly, and only clears on Reset. And if the condition is still present when Reset is released, the fault comes right back.