Limit switches: why NO causes problems in CNC — and why NC is the only serious choice

Limit switches are often treated as “simple inputs” in CNC machines.  
Two wires, a switch, done.

In reality, limit switches are part of the safety and reliability backbone of a CNC system.  Choosing the wrong type — or wiring it incorrectly — leads to false triggers, lost homing, random stops, and crashes that seem “mysterious”.

One of the most common mistakes is using Normally Open (NO) switches.

This article explains why NO switches cause real problems in CNC environments — and why Normally Closed (NC) is the only serious choice.

• Normally Open (NO)  
    The circuit is open when idle.  
    It closes only when the switch is pressed.
• Normally Closed (NC)
    The circuit is closed when idle.  
    It opens when the switch is pressed.

At first glance, NO looks simpler.  
In practice, it is the worst possible option for CNC machines.

In industrial control systems, the choice between NO and NC is not a matter of preference. Normally Closed (NC) wiring is the industry standard because it provides fail-safe behavior by design. When a system is idle, the circuit is closed and actively monitored.

Any fault — electrical noise, a broken wire, a loose connector, or power loss — forces the circuit to open and be immediately detected. With Normally Open (NO) wiring, the same faults remain invisible. This is why CNC machines, PLC inputs, safety loops, and emergency stop circuits all rely on NC contacts: they prioritize fault detection, noise immunity, and predictable behavior over apparent simplicity.

In CNC systems, reliability is not achieved through tuning or software workarounds. It is the result of correct design choices made early. Limit switches are not optional accessories — they are part of the machine’s safety and reference system. Choosing NC wiring is not about being conservative or “industrial enough”. It is about building a CNC machine that behaves predictably, detects faults immediately, and does not rely on luck to survive electrical noise. In serious CNC setups, NC is not a recommendation. It is a requirement.

For this reason, all CNC control boards and motion controllers designed by SourceRabbit use Normally Closed (NC) limit switch inputs by default. This is not a configuration preference or a firmware trick — it is a deliberate hardware design choice. NC wiring ensures immediate fault detection, higher noise immunity, and predictable behavior under real CNC operating conditions. The controllers are built around the assumption that limit switches are part of the machine’s safety and reference system, not optional accessories.