Description
Adding serial communication on existing micro USB power connector on a TP-link WR703N running OpenWRT.
WR703N: Low cost, looks good and runs Linux (you should get one : )
Development
A friend recently bought me a TP-Link WR703N in China for just 98RMB (that is 15$ or 12 €)!!! It is really an incredible piece of hardware including a 400MHz ARM (Atheros AR7240), Ethernet connector, 802.11 b/g/n wlan, USB port, 4MB flash and 32MB RAM. You even get a power adaptor and the device consumes just 0.5W. When it runs Linux (OpenWRT) I personally think that it is as or even more attractive than the 25$ raspberry PI. The only thing missing is access to the on board serial interface which I wanted to solve. Instead of adding a new connector to the case I simply used two of the available lines in the micro USB connector (used as the power connector for the router). This means that I can access Tx and Rx, GND and 5V thru the included USB cable. On the router there are two small copper colored test pads next to the ram (on the opposite side of the Ethernet connector). They are labeled TP_IN and TP_OUT. I connected the wires to the decoupling caps C55 and C57 next to the pads, since they were already tinned.
Thin enameled wires were soldered to connect the USB with the tx/rx pads.
I have already started to build a small PCB with a microprocessor, a mini USB connector for power (using the routers power adaptor with a standard USB mini cable) and a USB typeA female that provides power and serial connection to the router. The board has a lot of available I/O, an IR receiver and debug led’s. This board can be used to connect almost anything to the web. I think I will start with a 433MHz radio interface to control the lights at home.
BTW, the router is quite difficult to open. If you trim away a small part of the snaps, taking it apart is a lot easier next time.
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Very nice, except you somehow forget to tell exactly where the serial lines are available on the router PCB. I have looked at a photo showing the whole board, but I fail to find any obvious location for it. Would you mind telling where you found it?
Hi Nick
Good point, I agree that it is a bit difficult to see on the first photo…
You will find two small copper colored test pads next to the ram (on the opposite side of the Ethernet connector). They are labeled TP_IN and TP_OUT. I connected the wires to the decoupling caps C55 and C57 instead, since they were already tinned.
God luck and thanks for the comment!
Hi Johan,
thank you for the nice hint with the micro usb connector. I connected TP Out to pin 2 and TP In to pin 3. I’m not sure if this is correct because I cannot find a serial device on my laptop.
It would be nice if you can provide some more details. If it’s working it might be the best solution to get serial access to this tiny box.
Best Regards
Chris
Hi Chris
Adding the serial connection to the available USB pins in the power connector, doesn’t make it an USB device. It is still a serial connection. The advantage is that you get access to the serial connection without adding an extra connector (and cut hole in the router). To communicate to a pc, you need a serial to USB adapter; you can buy one or build one using a FTDI chip or reuse an old SEMC dock like this.
Please note that the serial connection on the router uses TTL-levels (5V signals), it is not RS-232 (12V signals) and you can’t use an USB to RS-232 adapter without limiting the signals.
Hi there just wanted to give you a quick heads up. The words in your post seem to be running off the screen in Ie. I’m not sure if this is a format issue or something to do with internet browser compatibility but I thought I’d post to let you know. The design and style look great though! Hope you get the problem fixed soon. Many thanks
Thanks for the tip. However I have not been able to recreate the problem you are mentioning in any of the web browsers I have tried. If you tell me which version of IE are you using and on what platform, I will see if I can fix it!
Johan von Konow, could you perhaps add some additional pictures of how you wired this up? It took me a bit to realize how awesome it is that you run the micro USB port as the serial out by wiring it in. I bricked my TL-WR703N and I’m working in getting console access and this is just what I need! I’m going to link this to Reddit’s in a couple of places because of the sheer genius of your idea.
Hi
I have added two pictures to show how tx/rx is connected to the USB port. It is still a bit difficult to see – tp_in is connected to pin 3 (from the left) on the usb port and tp_out to pin 4.
/j
Can I use your method to unbrick my wr703n?
Yes (hopefully). I bricked my router once during the process. I was able to restore the file system using a terminal connected to the serial lines.
/j