Xiaomi AC1200 - problems connecting through UART

I have a Xiaomi AC1200 (RA75). In order for it to receive OpenWRT, you have to connect to it via UART.

So I soldered a pin header to the pins.:

(from left to right: VCC, RX, GND, TX)

I am truly no soldering god, but from my point of view the result is not pretty, but it should work. I haven't had any problems with previous similar soldering jobs

Now I have used a CP2102-based USB2TTL adapter and connected it as follows: GND->GND, RX->TX, TX->RX, I have not assigned VCC.
I made the following connection settings in the device itself in the Windows Device Manager and in Putty (according to the device wiki):

  • Bits/s: 115200
  • Data bits: 8
  • Parity bits: None
  • Stop bits: 1
  • Flow control: none

I have set the COM port mentioned in the device manager.
Now I have booted the AC1200 and... nothing. :-/
I then swapped RX and TX and suddenly a lot of input came in, but unreadable gibberish. When I pressed a key, i.e. sent an input, I also immediately received a response in the form of more gibberish.

What I then did:

  • I measured the pins from the solder joint to the end of the cable with a multimeter - all ok.
  • I swapped the cable.
  • I tried TeraTerm instead of Putty
  • I tried Arch Linux instead of Windows (with minicom as client)
  • I used a different adapter (FTDI-based).

All gave the same result: I only get input if I swap TX and RX (i.e. TX->TX & RX->RX) and then it's just gibberish.

Am I doing something fundamentally wrong?
Where could I start to search?

Gibberish could mean wrong port speed.


Try Bits/s 120000

I played with the connection settings and once, when I set 7 data bits and lifted the device briefly, I actually got the correct output with the correct cabling (RX->TX).

So at first I thought it was the data bits. But since then everything has remained black.
Then I remembered that I had lifted the device and realized that it was a loose connection after all.
So I re-soldered the somewhat dubious-looking GND contact and lo and behold: it works!

In short: It was actually an incorrect solder contact.

Now I have a new little travel companion. My thanks therefore go both to the OpenWRT devs, who made the whole thing possible in the first place, and to the support from the community here, which is just as important for the project.

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