Black Horse F2301 NOT Gate Module – Output Stuck High Fault Analysis (Planar F System) - Excellent PLC

Fault Scenario

During functional testing of a safety sequence, the F2301 NOT gate module exhibited the following abnormal behavior:

Input signal: LOW → Output expected: HIGH
Observed output: constant HIGH, even when input transitioned HIGH
Module LED indicators: input recognized correctly, output LED ON constantly
Connected downstream safety interlocks remained engaged improperly

Symptoms suggested internal logic inversion failure rather than external wiring error.

Step 1 – Verify Input Signal Integrity

Measured input voltage: consistent with module specification
Input transitions clean: 0 → 24V and 24 → 0V
No signal bounce or slow rise observed

Inference: Input signal quality is not the cause.

Step 2 – Observe Output Behavior with Oscilloscope

OUTPUT_TEST:

Connect oscilloscope probe to output terminal.

Apply known LOW/HIGH input signals.

Observe waveform for inversion response.

Findings:

Output remains HIGH regardless of input
Minor ripple at output, consistent with internal relay coil energization

Conclusion: Output driver or relay mechanism likely stuck.

Step 3 – Inspect Internal Relay Mechanism

F2301 module uses small mechanical relays for logic inversion
Visual inspection and thermal test showed relay coil energized constantly
Mechanical contact partially welded due to prolonged duty cycle and high switching frequency

Observation: Module relay sticking prevented proper NOT operation.

Step 4 – Corrective Action

Replace F2301 module with spare unit
Verify new module passes functional inversion test:

FUNCTIONAL_TEST:

– Input LOW → Output HIGH

– Input HIGH → Output LOW

– Repeat 50 cycles

– Record response time and correctness

If replacement unavailable, careful cleaning and relay contact restoration possible (advanced repair, not recommended in safety-critical applications)

Step 5 – Preventive Recommendations

Avoid exceeding rated switching frequency for F2301 relays
Schedule periodic functional tests of logic modules
Keep module temperature within manufacturer specification
Consider upgrading to solid-state inversion modules in high-frequency applications

Engineering Insight

F2301 modules are robust but mechanical relays are wear components
Constant energization or high switching frequency increases risk of contact sticking
Proper preventive maintenance avoids unexpected safety interlock engagement failures

Conclusion:

If a Black Horse F2301 NOT gate module fails to invert signals properly, the most probable cause is internal relay sticking. Replacement or preventive maintenance ensures correct logic operation in Planar F safety systems.