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How-to-do-a-loopback-test.md

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How to do a loopback test
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A loopback test is a troubleshooting procedure to test the serial communication between the PC and Arduino board with a dedicated USB to Serial converter chip. It can be performed with the following boards:

  • Arduino UNO (classic)
  • Arduino UNO Rev3
  • Arduino UNO Rev3 SMD
  • Arduino Mega (classic)
  • Arduino Mega2560 Rev3
  • Arduino Mega ADK Rev3
  • Arduino Nano (classic)

Performing the test

  1. Disconnect the board from your computer or battery.

  2. Remove all shields, jumper cables, and other connections from the board.

  3. Connect a jumper cable from RESET to GND.

  4. Connect a jumper cable from Digital 0 (RX) to Digital 1 (TX).

    Arduino UNO with jumper cables connecting TX-RX and RESET-GND. Pin connections for Arduino UNO.

    Arduino Nano with jumper cables connecting TX-RX and RESET-GND. Pin connections for Arduino Nano.

  5. Connect the board to your computer and open the Arduino IDE or the Arduino Cloud Web Editor.

  6. Ensure that the correct port is selected.

    • Arduino IDE: Tools > Port > [Your Arduino Board]
    • Web Editor: Select your board using the boards dropdown.

  7. Open the Serial Monitor.

    • Arduino IDE: Tools > Serial Monitor
    • Web Editor: Select Monitor in the sidebar.

  8. Enter a message and click Send. Your message should immediately be echoed by the board, and appear in the output field below.

    Performing a loopback test in the Arduino IDE serial monitor.

    Performing a loopback test in the Arduino IDE serial monitor.

If the loopback test failed

A failed loopback test failed indicates that the USB to TTL serial adapter is damaged and that the board should be replaced. Contact us for questions about warranty, or visit the Arduino Store.

Note: This test will always fail for the CH340 USB to serial chip used on some derivate boards.

If the loopback test passes

A successful loopback test means that the board can communicate with the computer but is unable to upload or run the sketch. This can be the result of a missing or corrupted bootloader. You can burn a new bootloader using another Arduino board as an ISP programmer, see Burn the bootloader on UNO, Mega, and classic Nano using another Arduino.