Re-using a not so old TP-Link TL-WA901ND ver.4

I'm trying to re-use an old TP-Link TL-WA901ND ver.4 AP found in a garage ...

I've successfully installed the latest official OpenWrt FW release 18.06.9

but when I tried to come back to the official firmware (for performance testing purposes) this happen:

  1. Error on loading the original firmware from the WEB GUI

  2. No TFTP Recovery method (as documented here [OpenWrt Wiki] TP-Link TL-WA901ND)

For TFTP, rename with the '-factory' image to wa901ndv4_tp_recovery.bin

No way to put the right firmware back into the router flash; both the ip nor on my PC (TFTP Server) don't work.

Sniffing packets (TcpDump & WireShark) on the single router ethernet port shows no traffic at all.

  1. Native OpenWRT fail-safe recovery mode

works (to get an ssh shell on which to manually update FW)

Using a wired connection, trying first from LAN1 (only 1 port on my device), wait for the power-on sequence of lights that is specific to the device's bootloader then, when the fast 5-per-second blinking rhythm appear (during two seconds) quickly press any physical button; if

  • A faster, 10-per-second blink happen, failsafe mode was triggered
  • A slower, 2.5-per-second blink happen, normal boot continues (no failsafe sorry, retry)

[ the alternate method on which you've to press a button when a special broadcast packet is sent with text: “Please press button now to enter failsafe” and destination address port UDP 4919 doesn't work for me, no ethernet data at all ]

Then from a native shell, I wrote the official TP-Link firmware with :

mtd -r write tplink.bin firmware`

(tplink.bin is the original FW I've transferred via scp; 'firmware' is the mtd flash partition name to which wrote the binary image)

but I've done some mistakes :

1 + 256 = 1 [512 bytes] (firmware header struct/table) + 256 [128KB] (u-boot bootloader image)

  • when mtd finished with the error I rebooted the router :frowning:

If I had stopped for two minutes to think about what to do, maybe I would have realized that it was better to keep the router powered up and try something better than crossing my fingers and turning the router off and on again ...

But, It's no use crying over spilt milk.
So I'm trying to recover my bricked device via the (unpopulated) serial port

On v4 I need to bridge 2 points (near the TX & RX line) but, after having soldered the 2 points and the 4-pin connector I've only some garbage as output during the router booting process ... (115200 8N1 software flow control XON/XOFF)

I'm not so good in hardware so, before trying some tricks (pull-up resistor and so on...) I'm asking if anyone here has esperimented some troubleshooting about getting the right serial TTL signal on these TP-LINK devices.

Any suggestion ?

If Your router reboot itself in loop You should be fine after connecting serial cable . You should see this like loop disconnects on ethernet connection while connected to LAN port of tplink . If Only thing You have its always connected (or disconnected) ethernet no LEDs flashing You might have an brick .Bootloader programming might be needed in this case

Keep in mind that this is a 4/32 device, which can't even run contemporary -security supported- versions of OpenWrt at all, so probably adjust the intensity of your recovery attempts accordingly.