Fault Scenario

During safety system testing, the F2301 NOT gate module output failed to invert as expected:

• Input signal: LOW → Output expected: HIGH

• Observed output: stuck LOW regardless of input

• Input LED indicator: toggling correctly

• Downstream interlocks remained de-energized

Symptoms pointed to internal relay coil open circuit rather than logic failure.

Step 1 – Measure Coil Continuity

• Nominal coil resistance: 250 Ω

• Measured: ∞ Ω → open circuit

Inference: Relay coil is broken, preventing relay from energizing.

Step 2 – Inspect Module Power Supply to Relay

• 24V DC rail feeding relay: healthy

• Terminal voltage reaches relay pins during input transitions

• No voltage drop observed → external supply intact

Conclusion: Problem localized inside module.

Step 3 – Functional Verification with Oscilloscope

• Output remained LOW for all input transitions

• Confirms coil open → mechanical contact cannot actuate

Step 4 – Corrective Action

• Replace F2301 module (recommended in safety-critical applications)

• Advanced repair: replace relay coil, verify coil resistance, recalibrate module timing

• After replacement:

  • Input LOW → Output HIGH

  • Input HIGH → Output LOW

  • Output LED correctly reflects state

Step 5 – Preventive Recommendations

• Avoid exceeding rated switching frequency

• Monitor module temperature and vibration

• Conduct annual functional tests on logic modules

• Maintain spare F2301 modules for rapid replacement

Engineering Insight

• Relay coil failures are subtle: input LEDs may still function

• Output stuck LOW is often misdiagnosed as module logic failure

• Coil open circuit can occur due to thermal fatigue, mechanical stress, or manufacturing defect

Conclusion:

A Black Horse F2301 NOT gate module that fails to produce HIGH outputs usually has an open relay coil. Replacement and preventive testing ensure safe operation of Planar F safety systems.