Wait until the LED changes color, then release the reset button
The TFTP server should log the firmware file having been downloaded - if not, just keep trying
Once the file has been downloaded successfully, give it a minute or too, then try to get an IP address via DHCP
In case TFTP does not work, you'll have to attach a serial cable (see below). Serial login for stock firmware: root:sohoadmin
Note the serial port on the right part of the board, approx. in the middle. Four pins next to the two large capacitors.
The serial port pinout is TX, RX, GND, NC. (RX/TX might be swapped, I don't trust my usb-serial converter)
Port settings are 115200, 8n1, no flow control.
Note: The console output may have garbage characters in it, as well as it will not take input. In this case pulling TX high with a 15K resistor from 3.3 volt resolves this
Note: At least in v4.0 serial port uses lower voltage levels (3.3V), so regular serial adapters might not work correctly (even with added pullup-resistor). Use LVTTL USB to Serial adapter instead. The old Nokia CA-42 connector works fine (swap RX/TX between board and adapter), or use adapter based on PL2303HX chip.
Serial console login: root:sohoadmin
Debug output v1
Some debug output below on a WR940N v1.0 flashed via openwrt-ar71xx-tl-wr941nd-v4-squashfs-factory.bin image (MD5 d753155844ad30db6906781083192d1e). Note that it says WR741ND?!
system type : Atheros AR7240 rev 2
machine : TP-LINK TL-WR741ND
processor : 0
cpu model : MIPS 24Kc V7.4
BogoMIPS : 266.24
wait instruction : yes
microsecond timers : yes
tlb_entries : 16
extra interrupt vector : yes
hardware watchpoint : yes, count: 4, address/irw mask: [0x0000, 0x05f8, 0x0180, 0x0118]
ASEs implemented : mips16
shadow register sets : 1
core : 0
VCED exceptions : not available
VCEI exceptions : not available