Black Horse F2102 Safety Output Locked in SAFE State – Output Feedback Monitoring Fault Analysis - Excellent PLC

Fault Overview

During a scheduled restart, the Black Horse F2102 Safety-Related Application Module entered RUN mode successfully, but one safety output channel remained locked in SAFE (de-energized) state.

Observed symptoms:

Module in RUN mode
Three outputs functioning normally
One output permanently OFF
Diagnostic message: Output Feedback Error
No short circuit detected

Production equipment connected to that channel could not start.

Understanding Safety Output Monitoring

The F2102 safety module includes:

Redundant output driver stages
Output state verification circuit
External feedback monitoring (EDM)
Discrepancy detection between command and actual state

When the module commands an output ON, it verifies that feedback matches the expected condition.

If mismatch persists, output remains disabled.

Step 1 – Read Output Diagnostic Buffer

OUTPUT_DIAGNOSTIC:

Access channel diagnostics.

Identify feedback mismatch detail.

Record timing data.

Diagnostic message:

FAULT: OUTPUT_FEEDBACK_MISMATCH

COMMAND: ON

FEEDBACK: OFF

TIMEOUT: 200ms

The module detected no confirmation from external feedback loop.

Step 2 – Verify External Device Feedback Loop

FEEDBACK_LOOP_TEST:

Trace EDM (External Device Monitoring) wiring.

Check auxiliary contact wiring from contactor.

Measure continuity during activation.

Found:

Contactor auxiliary contact stuck open
Mechanical wear visible
Feedback loop never closed

Even though output driver energized, feedback signal did not confirm.

Why Output Remained Locked

Safety logic requires:

Command ON
Contactor closes
Auxiliary contact confirms closure
Feedback returns within defined time window

Because auxiliary contact failed, the module interpreted this as unsafe output state and locked channel OFF.

This prevents potential welded contact hazard.

Step 3 – Bench Test Output Driver

DRIVER_VERIFICATION:

Disconnect field wiring.

Measure output voltage at terminal.

Confirm driver stage functional.

Measured correct output voltage when activated.

Internal driver stage confirmed healthy.

Root Cause

Mechanical failure of external contactor auxiliary feedback contact caused persistent feedback mismatch, leading to safety output lock.

Step 4 – Corrective Action

CORRECTIVE_ACTION:

Replace defective contactor.

Verify auxiliary contact alignment.

Test EDM loop timing.

Reset module diagnostics.

After replacement:

Feedback signal returned correctly
Channel enabled normally
No further fault entries

Important Safety Note

Never bypass or short EDM feedback loop to force operation.

This defeats safety validation and violates compliance requirements.

Always repair the actual hardware cause.

Preventive Recommendations

Periodically inspect contactor auxiliary contacts
Replace components near mechanical life limit
Monitor contact resistance trends
Keep spare contactors in stock for critical circuits

Conclusion

When a Black Horse F2102 safety output remains locked despite correct RUN status, the most common cause is external device feedback failure. Proper EDM loop verification ensures safe restoration of operation in Planar F safety systems.