How to fix VESC killed by over current in serial comm?

I think your suppose is right, and you need to replace a new stm32 chip.

On the other hand, you should never connect the uart interface without common ground.

If both are dead, chances are the MCU is a toast. It's easy to check by trying to reprogram the MCU via the SWD (without connecting the main battery). If it takes the program and LEDs come alive, it may be good but the power supply is shot. Can be both. 

Check the 3.3v rail, those regulators do not like to be backfed with voltage, you may be lucky and only need to swap it out.

Power it and check any of the 3.3v pins, if you get nothing externally inject some power and see if you get signs of life.

Had a few flight controllers go this way from loose connections, worth checking the vin pin for said voltage, directly on the mcu as could even have fried a diode 

Image to the left shows the regulators,with the three pins at the top, the bottom right is the 3.3v output.

Good luck!

Hello everyone, I'm developing a project on basis of a VESC100/250, a battery supply 22S10P (4kWh) and a REX30 Rotex brushless motor, to run a paramotor. It worked well until today. The VESC is connected with an arduino mega2560, to manage infos from Vesc and display on oled. It worked as long as the arduino was powered by the Vesc 12V output. But as I was having for some bugs on my oled display, arduino being perturbated while the vesc was delivering power, I tried to change the power supply of the arduino to an external source...fatal ! The vesc didnot like it at all, I don't know exactly why, but the Vesc is not working anymore, even not connectable anymore ! I openned the warranted Trampa black box, but as I don't have any schematics or components layout of the Vesc100/250, I don't know how to fix and repair if possible ? Can any one of you guys help and support to fix this issue ? Thanks in advance.

Check the post right above yours. You might be lucky that only the controller's power supply blew up and it can be an easy fix. Check if the power supply outputs the expected voltage. Another thing to try, as I've mentioned above is to connect an SWD direct to the MCU to see if it comes alive. But yeah, using two separate supplies on two connected devices is a bad idea unless you use capacitive or optical isolation for the comms.  

Thanks Vadicus for your quick reply. Actually I opened the box, powered it with low voltage 16V battery on table, and checked all the voltages on the board, all are correct. I also checked the signals with the scope : the STM32 doesn't breath anymore ! Do you believe I may changed the chip by unsoldering it and weld a new one ? How to do it in this case, especially how to install the software in it (need for development tools, boatloader, etc.. ?) Or is it not a better way to chip back the board to Trampa for fixing it, even if warranty is gone ? Thanks 

If you get correct 5v and 3.3v, 12v voltages, then something else is burned. Most likely the MCU as you suggest. Sometimes, you get lucky and you can get the MCU back by reprogramming it. You'd need to find the SWD port and use an STlink to try to reprogram the chip with a fresh firmware. If that doesn't help, the MCU can be swapped with proper tools and skills. Obviously, you'd need to de-solder the old one and solder a new one, then reprogram. This of course, doesn't guarantee nothing else is burned and it all will work out. 50% that it will work, and 50% it won't.