Black Horse F2108 Multi-Channel Delay Conflict – Cross-Channel Timing Error in Planar F Safety Systems - Excellent PLC

Incident Overview

During a conveyor safety system test, the F2108 safety-related multi-function delay module showed unexpected behavior:

Two outputs intended for sequential activation overlapped
Output A triggered after configured 1 s delay
Output B triggered simultaneously instead of 3 s later
LED indicators suggested proper countdown individually, but real output timing conflicted
PLC logic confirmed input sequence was correct

Symptom indicated cross-channel delay interference rather than single-channel failure.

Step 1 – Verify Configuration of Multi-Channel Delays

Channel A: Delay 1 s
Channel B: Delay 3 s
Multi-function mode: Sequential output activation enabled
Cross-channel dependencies enabled in configuration

No misconfiguration detected; all parameters matched safety specification.

Step 2 – Observe Timing with Oscilloscope

OSC_PROBE:

Connect CH1 to output A terminal.

Connect CH2 to output B terminal.

Trigger module input.

Capture both channels simultaneously.

Findings:

Channel A: 1.02 s delay → correct
Channel B: 1.05 s delay → unexpectedly early
Channel B output pulse width normal, but activation window overlapped A

Step 3 – Investigate Internal Logic Dependency

F2108 internal design:

Multi-channel counters share common clock and debounce logic
Sequential mode relies on internal flag register to determine next channel activation
Race conditions can occur if input triggers are too close or inputs transition during debounce period

Observation: Input transitions occurred within 50 ms, causing flag register to reset prematurely → Channel B activated early.

Step 4 – Field Mitigation

Stagger input triggers by at least 100 ms to avoid flag race
Reduce wiring capacitance between channels to minimize cross-talk
Firmware validation: Check multi-channel sequential mode logic
Test multiple cycles under normal operation and peak load

After adjustment:

Channel A activated at 1 s
Channel B activated at 3 s
No overlap observed in 50 consecutive cycles

Step 5 – Root Cause Analysis

The cross-channel timing error was due to internal flag register misinterpretation caused by simultaneous input transitions and shared debounce circuitry
No hardware failure detected
Proper staggering of inputs and verifying sequential logic resolved issue

Engineering Recommendations

Avoid simultaneous input transitions for sequential multi-channel delays
Use oscilloscope to verify both input and output timing relationships
Document timing dependencies for commissioning
Consider separate modules if complex multi-channel sequences are critical

Conclusion

In the Black Horse F2108 multi-function delay module, sequential delay errors between channels can occur when inputs transition too closely together, triggering race conditions in internal logic. Understanding internal flag handling and input timing is essential to ensure safe and predictable multi-channel output sequencing in Planar F safety applications.