Hello! After sorting out my home networking cabinet, I power cycled one of my WDR3600s and then it stopped booting OpenWrt, instead flashing all the LEDs every two seconds or so.

I soldered a 4 pin header, found a USB to serial RS-232 cable with a QinHeng Electronics HL-340 as reported by lsusb, and plugged it into a RS232 to serial TTL cable.

I tried to connect with minicom at 115200 8N1 no flow control, but everything always shows up as garbage characters. What might I have done wrong?

Thanks in advance!

https://forum.openwrt.org/viewtopic.php … 87#p221987 !!!!!!

You need low-voltage 3V UART cable, not a "real" (high-voltage, bipolar) RS232. They're same at protocol logic, but it's about voltage levels. CPU talks plain 3V logic-level UART, it using 0V as logical zero and about 3V as logical one. Real RS232 uses bipolar voltages of +/-3 ... 12V. Needless to say, idea to put 12V signals at 3V CPU pins can burn CPU to the hell in worst case. This interface is NOT FOOL PROOF. It's internal/debugging thing for those who knows what they're doing. So check pinout and voltages TWICE before turning it on. Else you can PERMANENTLY destroy CPU in fatal way. As rule of thumb, when you deal with standard digital I/O pads it's really invalid idea to put voltages below ground plane level (aka 0V) or above of I/O power supply rail level (3.3V in case of most routers, but you better to check this twice, too).

And if you're 100% sure  that voltage level and pinout is correct, but there is still some garbage, check baud rate settings, stop bits and parity settings. Rather common setting is 115200 bps, 8-N-1 (8 bits, no parity, 1 stop bit). But it can vary and not set in stone. Messing these settings isn't anyhow fatal - it just warrants some funny garbage if settings of sender and receiver do not match.

(Last edited by t3st3r on 9 Jan 2014, 00:06)

Thanks a lot, I will check again when back home. This was a SamKnows router, and they did tell me to throw it away after I asked for a replacement so I figured I could give it a try.

Oh crap, I just measured the voltage for the rx and tx pins, and my multimeter says 5V. Going to get another one, see if it's 3.3V and then plug it in... hopefully the stuff isn't fried

EDIT: measured the wrong pins (3 and 4 instead of 2 and 3), RX and TX on the serial converter shows 0V. On the new one, it shows 6.6V on pins 2 and 3, and 13V on pins 3 and 4 o.O

(Last edited by zhongfu on 9 Jan 2014, 11:31)

well that does not explain garbage on rx side, usually a 5V ttl "hears" correctly 3.3V ttl.

have you tried pulling up tx pin of router to 3.3V like it's necessary in other models? leave rx pin disconnected for now...

http://img688.imageshack.us/img688/4219 … 193104.jpg

Please, have  tp-link 3600 bricked, have a low-voltage 3V UARTinterface and cable, need for debricking exact connection pin (interface and router....foto, how to...)

(Last edited by viocons72 on 3 Apr 2014, 18:52)