Contacts (input instruction)
XIC: Examine If Closed
Passes power when the bit it is examining is 1.
Also called: normally open contact, NO contact, examine on, -| |-.
XIC(tag)What it does
XIC is the instruction people mean when they say contact. It looks at one bit and asks a single question: is this bit a 1? If it is, power flows through and on to whatever is to its right. If it is a 0, the rung stops there.
The name confuses almost everyone at first, because it sounds like it is describing the state of a physical switch. It is not. XIC describes what the instruction does in the program: examine the bit, pass power if closed. What the field device is wired as is a separate question, and the two are deliberately independent. A normally closed pushbutton wired to an input still makes that input bit a 1 while the button is at rest, and an XIC on it will pass power.
Contacts in series are AND. Contacts in parallel are OR. That is the whole logic model, and almost every rung you will ever read is built from those two facts.
A rung you can run
Two conditions in series, one in parallel
[XIC(Start_PB),XIC(Motor_Run)]XIO(Stop_PB)XIC(Guard_Closed)OTE(Motor_Run)Start_PB or Motor_Run, and not Stop_PB, and Guard_Closed. The parallel branch around Start_PB is the seal-in: once Motor_Run is on it holds itself on, so the operator can let go of the button.
Run this rung in the simulatorWhat it is called on other controllers
| Platform | Equivalent |
|---|---|
| Rockwell (Studio 5000) | XIC |
| Siemens (TIA Portal) | Normally open contact, drawn -| |- |
| Mitsubishi (GX Works) | LD / AND / OR |
| CODESYS and IEC 61131-3 | Contact |
Common mistakes
- Using XIC on a stop button. A stop button is wired normally closed so that a broken wire stops the machine, which means its input bit is 1 while everything is healthy. The rung needs XIO on it, not XIC. This is the single most common and most dangerous beginner mistake in ladder logic.
- Assuming XIC means the field device is a normally open contact. It means the program is examining for a 1. The wiring is a separate decision.
Practise it
Interactive exercises that use XIC. 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.
- Sealed-in CoilA fill valve that opens on a button press and holds itself open until the tank-full sensor stops it. The seal-in coil is the building block behind almost every 'stays running until done' behavior.