Attempting to install DDWRT firmware on my TP-Link WiFi router, I finally managed to brick it. Once the router is bricked, it becomes as dead as the doornail. It does not power on, it is not detected by the PC and the reset button does not work. The only way to revive a bricked modem is either through the JTAG or through a simple USB-to-TTL adapter. The USB-to-TTL adapter basically provides a convenient way of connecting your PC to a device through the TTL serial interface. This type of interface is used by many electronic devices that have flash memory on the NAND or EMMC chips, e.g., routers, modems, hard disks, mobile phones and so on.
I bought a cheap USB-to-TTL adapter from eBay and tried to use it to resurrect my bricked router, but nothing happened. I thought it would be a good idea to first check if the USB-to-TTL adapter is faulty or not (yes on eBay, you never know). So here is a simple way to use the loopback test to find out whether your USB-to-TTL adapter is working fine or not.
- Use a jumper wire and connect the RXD pin to the TXD pin of your USB-to-TTL adapter and plug it in to one of the USB ports of your Windows PC.
- Press Win+R to open the Run dialog, type devmgmt.msc in it and press Enter.
- In the Device Manager, expand Ports (COM & LPT) and check the port number assigned to your USB-to-TTL adapter. In my case, it is COM22. It can be anything depending on the COM ports availability on your PC.
- Download uCon software and install it on your Windows computer.
- Launch uCon and select Serial Backend. Choose the Com port as you found out in the step 3 above and click OK.
- Now another uCon window shall open up. Here you can send data to the serial port. Start typing anything you like. If the USB-to-TTL adapter is working, whatever you type should be echoed back (it should be visible to you).
- Now remove the jumper cable and try typing in uCon window. Nothing should be echoed back now.
The loopback test for USB-to-TTL adapter connects RXD and TXD pins together, so that the data you transmit is received back by your PC. This is why the characters you type, should be echoed back to you for a working adapter. If it is faulty, then either it is not able to send or not able to receive the data – in both cases the loopback test fails.