HC-950 How to do a loopback test (#22)

* Complete list of compatible boards, pin diagrams, and post-test instructions.

* Increase visibility of pin connections

* image alt text

* Added note about CH340

Author: seaxwi

content/Hardware/Generic/How-to-do-a-loopback-test.md

@@ -2,19 +2,17 @@
 title: "How to do a loopback test"
 ---
 
+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:
 
-For Arduino boards with a dedicated USB to serial adapter chip (Uno, Mega, or Nano), a *loopback test* can be performed to test the serial communication between the board and your PC.
+* Arduino Uno (classic)
+* Arduino Uno Rev3 <!-- [X] Tested 2022-03-24 -->
+* Arduino Uno Rev3 SMD <!-- [X] Tested 2022-03-24 -->
+* Arduino Mega (classic)
+* Arduino Mega2560 Rev3
+* Arduino Mega ADK Rev3
+* Arduino Nano (classic) <!-- [X] Tested 2022-03-24 -->
 
----
-
-A failed loopback test indicates a problem with one or more of the following:
-
-* The host computer
-* Hardware driver
-* USB cable
-* The board's USB to Serial converter
-
-## Instructions
+## Performing the test
 
 1. Disconnect the board from your computer or battery.
 
@@ -24,6 +22,12 @@ A failed loopback test indicates a problem with one or more of the following:
 
 4. Connect a jumper cable from **Digital 0 (RX)** to **Digital 1 (TX)**.
 
+   ![Arduino UNO with jumper cables connecting TX-RX and RESET-GND.](img/loopback-circuit-uno.png)
+   _Pin connections for Arduino UNO._
+
+   ![Arduino Nano with jumper cables connecting TX-RX and RESET-GND.](img/loopback-circuit-nano.png)
+   _Pin connections for Arduino Nano._
+
 5. Connect the board to your computer and open the Arduino IDE or the [Arduino Cloud Web Editor](https://create.arduino.cc/editor).
 
 6. Ensure that the correct port is selected.
@@ -34,8 +38,30 @@ A failed loopback test indicates a problem with one or more of the following:
    * **Arduino IDE:** *Tools > Serial Monitor*
    * **Web Editor:** Select *Monitor* in the sidebar.
 
-8. Enter a message and click Send (the baud rate does not matter). Your message should immediately be echoed by the board, and appear in the output field below.
+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.](img/serial_monitor_loopback_test.gif "Performing a loopback test in the Arduino IDE serial monitor")
 
-  *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](https://www.arduino.cc/en/contact-us/) for questions about warranty, or visit [the Arduino Store](https://store.arduino.cc/).
+
+> **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 is able to communicate with the computer but is unable to upload or run the sketch. This can be the result of a missing or corrupted bootloader. If you have a second board, you can use one of these guides:
+
+* [How to burn the bootloader between two Arduino UNO](https://support.arduino.cc/hc/en-us/articles/360012048080-How-to-burn-the-bootloader-between-two-Arduino-UNO)
+* [How to burn the bootloader between two Arduino Mega](https://support.arduino.cc/hc/en-us/articles/360012048060-How-to-burn-the-bootloader-between-two-Arduino-Mega)
+* [How to burn the bootloader on an Arduino UNO using an Arduino Nano](https://support.arduino.cc/hc/en-us/articles/4411327104402-How-to-burn-the-bootloader-on-an-Arduino-UNO-using-an-Arduino-Nano)
+* [How to burn the bootloader in an Arduino Nano using an Arduino UNO](https://support.arduino.cc/hc/en-us/articles/360012048100-How-to-burn-the-bootloader-in-an-Arduino-Nano-using-an-Arduino-UNO)
+* [How to burn the bootloader on an Arduino Uno using an Arduino Nano](https://support.arduino.cc/hc/en-us/articles/4412950859666-How-to-burn-the-bootloader-on-an-Arduino-Uno-using-an-Arduino-Nano)
+* [How to burn the bootloader on an Arduino Mega using an Arduino UNO](https://support.arduino.cc/hc/en-us/articles/4408893470610-How-to-burn-the-bootloader-on-an-Arduino-Mega-using-an-Arduino-UNO)
+* [How to burn bootloader to Arduino Uno using Arduino Mega](https://support.arduino.cc/hc/en-us/articles/4412950855442-How-to-burn-bootloader-to-Arduino-Uno-using-Arduino-Mega)
+
+Additional resources:
+
+* [How do I burn the bootloader to my AVR board?](https://per1234.github.io/ino-troubleshooting/burn-bootloader.html#burn-bootloader)