Ender 3 Pro USB (ch340) burn?

Hello, I'm asking here for some directions becouse some of the posts in this forum helped me in my project. I'm using ESP8266 to communicate with ch340 directly.

  1. I'm powering the esp8266 from different power supply than the ender (I suspect this was the problem)
  2. I'm monitoring the process and send some commands to printer periodically, had no problems.
  3. I'm turning on/off the printers power supply when needed through a relay that is controlled from the esp8266

Everything was fine for about a week, but from yesterday I've got no data comming to ESP8266.
I've checked if the problem is in the ESP, but it works fine.
One thing that points me to think that the CH340 has gone bad is that I'm not able to send, nor receive anything by connecting it to PC by USB.
Am I on the right direction?
I want to be sure before I replace the CH340.
Q 1: What should I check for when connect USB to PC? I'm trying to read/write to the port thorugh serial terminal (I see the ch340 recongnized in device manager, I'm connecting to the port).
Q 2: If the pins (or the ch340 chip) has been burned, how did this happen? The ESP8266 is powered by 3.3V, as the CH340 on the Ender 3.
Q3: Am I right to think that when the ESP8266 is powered and the printer isn't, there's a power flow through the TX/RX pins of the Ender3 and this caused the problem? - If so, how to limit the current or prevent this at all?

I can't speak for anyone else in this forum, but without some additional details / links to the original directions, I couldn't provide any assistance.

I use a smart plug to turn the AC power on/off to my LulzBot TAZ 6 and control it with OctoPrint-TPLinkSmartplug.

Yes likely, if you can't comm like you did before your mod, you probably broke something.
You say you used a seperate power supply for the ESP8266. The image you linked to shows the Grounds being tied together, did you do that or did you leave the grounds not connected together? If you did not tie the grounds together, this is a good way to burn out a chip.

Well you should get something out of the printer pretty continuously. But sending it a M115 should cause it to return a lot of detail. This is a good way to see if it is just idle or if it just is not communicating.

See my initial comment above. If the grounds are not tied togeahter, each side will see the comm lines as a floating level... It would cause a much higher voltage to be applied. Hard to predict what the outcome would be.. but it would not be good in most cases. You likley just go lucky that it worked at first and that the chip generally is a little forgiving with levels above 3.3 maybe up to 5v but over a week or so of having to displace all that heat. Its just a matter of time before failure.

Ya, there is going to be something for sure. To help keep it from being an issue, be sure to tie the grounds together but a better way to do it would would be to use an opto isolator for the signals. If you have to connect like this. I am not sure what else you are doing with the Esp but the relay things... There are better ways to control a relay than your current solution and it would not require anything more than you already have.