I am trying to flash Wemos D1 mini (ESP8266) using NodeMCU according to this video. Pins NodeMCU → Wemos:

  • RX → RX
  • TX → TX
  • D3 → D3
  • RST → RST
  • 3.3V → 3.3V
  • GND → GND
  • EN → GND

When flashing Wemos LED is blinking (probably as a response for reset), but flash process does not start. VS Code writes to log the following:

Uploading .pioenvs\esp8266\firmware.bin
esptool.py v2.6
Serial port COM11
A fatal error occurred: Failed to connect to ESP8266: Timed out waiting for packet header
*** [upload] Error 2


  • I flash Wemos because there is a problem with flashing it using build-in USB (one transistor was not soldered by error). Hence I use NodeMCU as USB-TTL converter.
  • There is no problem in USB cable. I used it successfully to flash another Wemos board (which has no defects) using VS Code (same procedure/method).
  • I tried to power Wemos with a separate power, but that does not help.
  • Similar question: ESP-01 module unable to flash or communicate – suggests wire 3.3V → RST (Wemos).

What can I try additionally?

Update on Dec 2022:

Just want to confirm that D1 Mini can be flashed with external USB-TTL converter by connecting 3.3V and GND to D1 Mini, and TX to RX, RX to TX (cross link). D3 (GPIO0) should be grounded before D1 Mini is started e.g. before powering USB-TTL converter / connecting USB.

enter image description here enter image description here enter image description here

  • 2
    \$\begingroup\$ You must find a better way of making connections to the D1 board. I don't think there's any point in talking about other sources of errors until that gets fixed. \$\endgroup\$ Feb 1, 2021 at 1:43
  • \$\begingroup\$ Your video seems for ESP8266-01, which is very old. Is your D1 Mini 12? \$\endgroup\$
    – tlfong01
    Feb 1, 2021 at 3:41
  • \$\begingroup\$ I have spent some 100 hours testing out ESP8266 (mostly -12, less -01) There are many tricks you need to be aware of. I have a blog of about 100+ posts sharing my newbie learning experience. You might like to browse my posts: (1) Testing ESP8266-01/12 - rpi.org.forum 2108sep29, raspberrypi.org/forums/… (2) rpi.org.forum Search found 155 matches: esp8266 tlfong01 Searched query: esp8266 tlfong01, raspberrypi.org/forums/search.php?keywords=esp8266+tlfong01. Happy learning. Cheers. \$\endgroup\$
    – tlfong01
    Feb 1, 2021 at 3:48
  • \$\begingroup\$ after you make the wires have real contact, you have to put the NodeMcu esp8266 in flashing mode. use esptool command line switches to suppress resetting of the Wemos D1 to flashing mode \$\endgroup\$
    – Juraj
    Feb 1, 2021 at 6:10
  • \$\begingroup\$ I assume you are using the following to flash your ESP8266-12. (1) D1 Mini ESP8266-12, (2) ESPTool v2.6. You other question is on ESP8266-01 which, as I said, is very out of date and had tricky problems already sorted out in 12 models. \$\endgroup\$
    – tlfong01
    Feb 1, 2021 at 6:21

1 Answer 1


rx -- tx

tx -- rx

et non

rx -- rx

tx -- rx

  • \$\begingroup\$ English only please \$\endgroup\$ Oct 7, 2023 at 23:19
  • \$\begingroup\$ Believe me, it's not about RX-TX wiring. D3 (GPIO0) should be grounded before the board is started. The one can use external USB-TTL converter to flash it. \$\endgroup\$
    – dma_k
    Oct 8, 2023 at 21:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.