Persistent flicker on SRx v4.01 Ports 2/3 with KULP 32 FPP 8.4.1 – test results

[ricardo]

Hi all,
I'm chasing a stubborn flicker issue and would appreciate a sanity check. Here's the setup and everything I've tested, step by step.

Hardware/Software

• Transmitter: FPP on Kulp32-AB
• Receiver: Falcon SRx PSU v4.01
  • Smart Receiver mode v2
  • Receiver A (DIP switches 1–4 ON)
  • Single receiver in chain
• Controller software: FPP v8.4.1 (v8.4 branch)
• Pixels: WS281x
• Config pushed from xLights

Symptoms

• SRx Port 1 and Port 4: rock solid
• SRx Port 2 (and previously Port 3): heavy flickering
• Any pixel string that works on Port 1/4 will flicker when moved to Port 2/3
• Cable lengths and strings are unchanged between tests

Tests performed

• Eliminate pixel/cable variables
• Swapped pixel strings between ports without changing cables or lengths.
• Result: issue follows the SRx physical port (2/3), not the pixels or cabling.
• Per‑port configuration parity
• Verified per‑port settings identical in FPP/xLights: WS281x timing, same global controller options.
• Tried "slow" timing mode for WS281x on all ports.
• Result: no improvement on Port 2/3.
• Addressing and ranges
• Ranges/universes created and uploaded from xLights; verified continuous, no overlaps.
• In FPP Display Testing, enabled each port's range in isolation.
• Result: Port 2 still flickers even when driven alone.
• SRx internal test pattern
• Stopped FPP output.
• Enabled SRx internal test pattern.
• Result: all four SRx ports run clean with the internal pattern.
• Logical port swap (software remap)
• In FPP, swapped the channel ranges for Port 1 and Port 2 (1 ↔ 2), keeping all physical wiring unchanged.
• Also tested each swapped range in Display Testing separately.
• Result: flicker remains on the same physical connector (Port 2), even when driven with Port 1's logical range.
• Power and reboots
• Full power cycle of KULP32 and SRx (10–15 s off).
• Result: no change.
• Connector check
• Swapped Phoenix plug assemblies between ports to rule out connector issues.
• Result: fault does not follow the plug; remains on SRx Port 2 (and earlier, Port 3).

Current assessment

• Since the SRx's internal test pattern is clean for all ports, but any data from the KULP 32AB/FPP path causes flicker only on the same physical SRx port, this points to a problem on the differential data path for Port 2 (and possibly Port 3) between the KULP 32AB and SRx. The issue does not appear to be pixel type, timing, addressing, or cabling to the pixels.

What I'm looking for

• Has anyone seen a SRx v4.01/KULP 32AB case where the internal pattern is fine but external data flickers on a specific receiver port?
• Any known firmware quirks for SRx v4.01 or KULP 32AB that affect ports 2/3 specifically?
• Recommended additional diagnostics to pinpoint whether the fault is on the KULP 32AB transmitter channel vs. the SRx receiver channel for Port 2 (e.g., suggested cross‑patch tests on the differential outputs)?
• Before I RMA: any other settings to check for Smart Receiver v2 chains with a single receiver (Receiver A) that could cause this symptom?

Temporary workaround

• I can run on SRx Ports 1 and 4 for now, but I'd like to restore Ports 2/3 if possible.

Thanks in advance for any pointers or confirmation that this is likely a hardware channel failure on the differential link.