Herelink Air Unit v1.0 UART RX possibly exposed to 5V for hours – is RX 5V tolerant? How to test / repair?

HereLink
1

Hello everyone,
I’m troubleshooting a Herelink Air Unit v1.0 telemetry UART issue and I suspect the air unit UART RX pin may have been exposed to 5V due to a wiring mistake.

Setup

  • Flight controller: Cube Orange+
  • Connection: TELEM2
  • Device: Herelink Air Unit v1.0
  • Goal: MAVLink telemetry between Cube and Herelink

What happened (possible wiring fault)

I mistakenly plugged the TELEM2 connector one pin offset on the Cube side. The cable also had a broken section, and I didn’t realize the full wiring state at the time.

As a result, the Herelink Air Unit RX line might have been connected to the Cube’s +5V pin for a couple of hours.
After I discovered the issue, I repaired the cable (re-soldered) and connected everything correctly again, but there is still no communication (no MAVLink link).

Questions

  1. Is the Herelink Air Unit v1.0 UART RX pin truly 5V tolerant?
  • I’ve seen references that v1.0 UART supports 3.3V/5V TTL, but I’d like confirmation from CubePilot/maintainers or anyone who has measured/verified it.
  1. If RX is 5V tolerant for logic, does that still apply if RX is tied to a constant 5V rail for a long period (hours)?
  • Could that stress the input protection (clamp diode, level shifter, etc.) and kill the port?
  1. Because the connector was offset, it’s also possible that TX-to-TX or other pin conflicts occurred temporarily.
  • In such a miswire case, which component is more likely to fail: the Air Unit UART interface or the Cube TELEM2 port?
  1. What’s the best way to test whether the Air Unit UART port is still alive?
    For example:
  • Any known LED behavior on the air unit that indicates telemetry RX activity?
  • Recommended loopback test method (if possible on the air unit side)?
  • Any way to verify at the Linux/Android level (logs, serial device nodes, etc.)?
  1. Does anyone know what UART/level-shifter/interface chip is used on the Herelink Air Unit v1.0 telemetry port?
  • If identifiable, I can check the datasheet absolute max ratings and whether it’s realistically repairable.
  1. Is component-level repair feasible?
  • Has anyone replaced the UART/level-shifter chip on the Air Unit board (hot air / rework), or is the recommended path typically RMA / replacement?

Additional info I can provide

  • Cube TELEM2 parameters (SERIAL2_*), baud rate, protocol, etc.

  • Pictures of the connector pinout and the cable (if useful)

  • Scope readings on Cube TX line (if that helps confirm the FC is transmitting)

Any guidance would be appreciated — especially confirmation about 5V tolerance, likely failure mode, and practical diagnostic steps.

Thanks!

2

Hi again, I did some additional testing with a USB–TTL (3.3V) adapter to verify whether the FC and the Herelink Air Unit UART lines are alive.

1) FC TX looks active

  • I connected Cube Orange+ TELEM2 TX + GND to the USB–TTL adapter (RX side).

  • The adapter’s RX activity LED was blinking (turning on/off), which suggests the FC TX is actively outputting data (likely MAVLink).

2) Air Unit TX also looks active

  • I then connected Herelink Air Unit TX + GND to the same USB–TTL adapter.

  • The adapter’s RX activity LED was blinking again, suggesting the Air Unit TX line is also outputting something.

3) Hardware changes

  • I replaced the JST-GH connector on the Air Unit side (to eliminate a bad connector/contact possibility).

  • Even after this, LED1 on the Air Unit does not show orange/yellow telemetry indication (it used to indicate FC data reception).

Question / Hypothesis

Given that both FC TX and Air Unit TX show activity, is it possible that:

  • the Air Unit’s UART RX path is damaged (so it cannot receive from the FC),
    while the TX path still works?

In other words: **can the UART input (RX) fail independently while TX still toggles and appears alive?
**
If RX is dead, is there any realistic component-level repair path ?

3

Just confirming, you have the wires:
Tx to RX and Rx to TX
CTS to CTS and RTS to RTS.

and paramaters:

BRD_SER2_RTSCTS,2
SERIAL2_BAUD,115
SERIAL2_OPTIONS,0
SERIAL2_PROTOCOL,2
4

Hello Shawn,

The wiring was OK — I found the root cause. Applying 5V to the Air Unit RX (Cube TX) seems to have damaged the RX input circuitry. I’ve seen claims that the RX pin is 5V-tolerant, but in my case it appears it was not.

For troubleshooting, I measured the following:

  • Air Unit RX wire (Cube TX)

    • Disconnected: ~1–3 V on the multimeter

    • Connected: drops to ~0.5–1 V on RX

  • Air Unit TX wire (Cube RX)

    • Disconnected: ~1–3 V

    • Connected: ~1–3 V (no significant change)

So the Cube side looks healthy and is driving the line as expected. The Air Unit RX input is likely damaged (loading/pulling the line down once connected). I’ve already sent the unit for repair.

Could you please DM me with guidance on what typically fails in this situation and what components should be replaced/checked on the RX input path? I can forward your notes directly to the repair technician.

Also, while disassembling the RF cap, I accidentally lost two tiny 0201 components on the RF side (I didn’t realize the UART circuitry was on the opposite side). If you can share the component values/part IDs, I’d really appreciate it.

Thanks in advance,

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