Serial Connection TL-WR841 unclean serial output

For Developers
1

I am trying to set up TL-WR841 HW version 8.4 serial connection. However the output is somehow unclean.owrt

What errors should I look for in this case?

Already tried to desolder the pins and clean the board, but the output is still the same.

2

Assuming that you've got the right serial speed selected and that the logic level on your serial adapter is compatible with the router, check for appropriate pull-up/-down on the lines.

3

There are two empty resistor positions on the board near to RxD, might be that the issue? They were originally unpopulated...

4

Quite possibly. I know a pull-up is needed on my Archer C7s. Notes for that specific model at https://openwrt.org/toh/tp-link/archer-c5-c7-wdr7500#recovery_using_serial_connection

5

owrt2

The output is now cleaner, but I am still unable to send keystrokes to the serial port.... Maybe another resistor value is needed?

6

I just checked the serial adapter with my Arm board and it is working...

7

I had working many 841v8, it's no need any extra resistor.
Try reconfigure serial adapter setting with 57600 8N1 flow hardware off. Or try another console software e.x: minicom, putty windows.

8

Do you connect the VCC as well?
The baud rate is definitely 115200 as with other baud rates I am not getting any readable text.

9

^@ is the shell escape sequence for 0x00, for the 'y' characters I'm not sure whether it is just missing the dots (0xFF) or it's actually y (0x79).

However it still looks like are hardware issue, some more things you could try:

  • desolder the RX LED if you have one of those USB-UART converters (CP210x etc.), those bright LEDs can sometimes pull the line very high as they will use that RX line level as their only power supply...
  • check the pinout again, maybe your receiver is very sensitive and you're actually just receiving the crosstalk induced from the neighbor pin
  • just connect GND and RX at first until you have clear reception, only then add the TX line of your USB-UART, never connect 3V3
10

I just took a voltmeter to measure voltage between VCC and GND, it shows 0V, which is not good.

11

Don't connect 3.3v pin. Try measure Tx pin of router >= 2.5v, change another power supply if below.

12

It turned out there was a bad soldering joint on GND pin. So I desoldered the socket and soldered wires instead. Now it is working as expected. You can mark this as solved.

13

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