Possible UART0 Failure

I believe the UART that USB and telemetry share has stopped working.

Everything was working great for months with telemetry, teensy, jd-miniosd, all sending data to their respective destinations (pc, taranis, goggles). Now there is no mavlink. Power cycled APM and miniosd and still no data transfer.

Connected only the telemetry radio and both radios have a solid green light but neither have the activity leds flashing and cannot connect via MP or APM planner. Removed all peripherals and plugged APM directly to pc via USB and although the leds show it is booting properly I either get Unknown Device and the Windows sound effect of USB connecting/disconnecting repeatedly, or windows says the USB device has malfunctioned. Have tried both pc’s that I use normally, and even my gf’s pc that has never had MP installed on it and same issue.

Tested output of power brick to APM and is 5v.

Tested at I2C and 3.3v regulator is working.

Serial/PPM Tx led is continuously flashing with nothing but USB plugged in, Rx does not light up.

Have uninstalled the 3dr drivers, MP, and rebooted before reinstalling and same issue. Tried 3 different USB cables, tested them with my teensy and telemetry radios and the cables are good. If the radios are connected to the apm and pc respectively, MP can query both radios and change/save settings wirelessly.

Powering APM via brick and having servos on the output I can still move them like normal, booting the APM up goes through the normal servo test mode, and I can use my taranis to change modes, but there is no data coming out of the telemetry port. The fix led on GPS flashes blue and the red led is solid.

I let it sit overnight hoping it was as simple as a thermal fuse or something that would reset itself but 12+ hours later it’s still a no-go. Gear has never been flown, and I didn’t change anything in between the time it worked and it’s failure (charged batteries).

All 3 pc’s are Windows 7 Home Premium 64bit

3DR APM 2.6 (JP1 is Open) (Input 2-3 are jumped on signal lines) (Output 1-4 connected to servo/esc)

3DR 915MHz Telemetry 2nd Gen (SiK 1.9.1)

3DR uBlox LEA-6H (UART1/I2C)

3DR Airspeed Kit (A0)

3DR Power Brick

AP 3.0.1 Stable

MP 1.3.0

PJRC Teensy 3.1 w/Mavlink FrSky-S.Port v1.3 (Distribution Board)

jD-MiniOSD 1.2 w/MinimOSD-Extra v2.4 (Distribution Board)

jD-Telemetry Distribution Board (Telem)

FrSky S.Bus-CPPM Converter (Input 1)

MachadoLabs RSSI-PWM Converter (A1)

FrSky X8R (Input 1 via converter)

My guess would be that the MUX chip is fried. I had this about a year ago. There’s 3 or 4 options:

1.) Get a new MUX chip from an electronics supplier and solder it on yourself. NOT easy! Needs a very steady hand and SMD soldering experience. But doable.
2.) Send the APM to 3DR for repair
3.) Send the APM to somebody else for repair (if I remember right, somebody on DIYD was offering exactly this kind of repair)
4.) Use the APM without telemetry and USB.

There is a trick to still get connectivity - if you open the case and connect an FTDI directly to the UART0 tabs on the border of the board but that’s a bit cumbersome to do all the time.

That is what I figured. I will call 3DR for a quote on the repair. Thank you.

Purchased a PixHawk @ accompanying airspeed kit yesterday anyway. Getting too close to being able to fly this rig to wait.

Here are some pics of StephanG’s suggestion to bypass the MUX and go directly to the UART0 pins on the side of the APM. It Works!!! Successfully flashed the firmware and can get telemetry out of it, albeit @ 57600 baud. Thanks again StephanG.