Normally Closed (NC) Contact
Quick answer
A normally closed ladder instruction is true when its referenced bit is 0 and false when the bit is 1. That logical test must not be confused with a physical normally closed contact, whose state is defined for the device in its normal, de-energized condition.
Key Takeaways
- A normally closed ladder instruction is true when its referenced bit is 0 and false when the bit is 1. That logical test...
- Beginner-level topic in Ladder Logic Elements
- Commonly used in: Testing an off or false Boolean condition, Stop and permissive logic with documented signal conventions
- Related to: Normally Open (NO) Contact, Ladder Logic, Coil (Output)
Detailed Definition
In ladder logic, a normally closed-style contact examines a Boolean value for the off or false state. Rockwell calls this XIO (Examine If Open); other platforms use a negated contact symbol. It does not force a field device open or closed—it only evaluates a bit.
A physical normally closed switch or relay contact is a wiring property. The PLC input value also depends on wiring, module configuration and whether the device is energized. For safety functions, use the approved safety architecture and validation process rather than assuming that a normal PLC contact symbol creates a safety function.
Evidence and review status
Logical behavior is described using common IEC-style and Rockwell terminology. Physical fail-safe and safety behavior must be established from the electrical and safety design.
Technical review date:
Critical behavior to understand
- The ladder symbol evaluates a stored Boolean state; it does not describe the field contact by itself.
- XIO is true when its addressed bit is off and false when the bit is on.
- A broken wire may look like an active stop only when the circuit and diagnostics were designed that way.
- Standard PLC logic is not a substitute for a safety-rated input, logic solver and output path where required.
Verification checklist
- 1Write down the physical contact state with the device healthy and de-energized.
- 2Verify the PLC input value in each field condition.
- 3Confirm the rung result for bit values 0 and 1.
- 4Test wire break, power loss and reset behavior under the approved procedure.
Worked example
Separate a physical NC stop from an XIO instruction
A healthy wired NC stop loop energizes input StopHealthy, so the PLC sees StopHealthy = 1 during normal operation.
- 1Observe the input when the stop loop is healthy.
- 2Operate the stop and confirm the input changes to 0.
- 3Choose ladder logic based on the named meaning StopHealthy, not the drawn field contact alone.
Expected result: The documented signal meaning makes the rung readable and exposes a wire-break or power-loss state. Safety adequacy still requires the approved safety design.
Primary and technical sources
These links support the specific behavior or product identity described above. Project documentation and the target platform's help remain authoritative for implementation.
- XIC and XIO instruction summary
Rockwell Automation
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Common Questions
What is Normally Closed (NC) Contact?
A normally closed ladder instruction is true when its referenced bit is 0 and false when the bit is 1. That logical test must not be confused with a physical normally closed contact, whose state is defined for the device in its normal, de-energized condition.
When should I use Normally Closed (NC) Contact?
Normally Closed (NC) Contact is particularly useful in scenarios such as Testing an off or false Boolean condition and Stop and permissive logic with documented signal conventions. Consider implementing it when you need reliable, efficient solutions for these types of applications.
What should I verify before using Normally Closed (NC) Contact?
Write down the physical contact state with the device healthy and de-energized. Verify the PLC input value in each field condition. Confirm the rung result for bit values 0 and 1. Test wire break, power loss and reset behavior under the approved procedure.
What are related concepts I should learn?
To fully understand Normally Closed (NC) Contact, you should also familiarize yourself with Normally Open (NO) Contact, Ladder Logic, and Coil (Output). These concepts work together in industrial automation systems.
Continue Learning
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Quick Info
- Category
- Ladder Logic Elements
- Difficulty
- Beginner
- Tier
- Important
About Ladder Logic Elements
Contacts, coils, timers, counters, and ladder diagram components