> For the complete documentation index, see [llms.txt](https://www.multi-module.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://www.multi-module.org/advanced-topics/serial-debugging.md).

# Serial Debugging

Serial Debugging can be used as a diagnostic tool for advanced troubleshooting of the MULTI-Module.

{% hint style="info" %}
Serial debugging is only practical with external MULTI-Modules.
{% endhint %}

**Step 1:**  Upload a serial debugging firmware build to your MULTI-Module.

| Module Type                                       | Debug Build | Constraints                                                                                                                                                                            |
| ------------------------------------------------- | ----------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| iRangeX, Banggood, older Jumper JP4IN1            | usbdebug    | If you have to unplug or power-cycle the module, you must close and re-open the serial monitor within 30s.                                                                             |
| Newer Jumper JP4IN1                               | ftdidebug   | If you have to unplug or power-cycle the module, turn the radio on with the USB cable unplugged from the computer, then plug the cable in once the module has started (red LED is on), |
| Any module with an external USB-to-serial adapter | ftdidebug   | If you disconnect the USB-to-serial adapter from the computer you must close and re-open the serial monitor.                                                                           |

{% hint style="success" %}
If you use Flash Multi to upload the debugging firmware to your module, the application will ensure that the firmware you are upload matches what your module requires.
{% endhint %}

**Step 2:**  Run Flash Multi (see the [Firmware Update Methods page](/using-the-module/firmware-updates.md)) and open the Serial Monitor.

**Step 3:**  Read the output, copy and paste, or save it to a file

![Flash Multi Serial Monitor with Output](/files/-LyB1PKe3RbaRdnVEwsn)

{% hint style="danger" %}
Before you use your MULTI-Module for flying you should revert to a non-debug firmware.
{% endhint %}