Follow these sequential steps to resolve the serial connection failure.

Most Set-Top Box logic boards run on 3.3V TTL levels . Ensure your adapter toggle switch is explicitly set to 3.3V rather than 5V to protect the board from hardware damage. 2. Isolate and Test the Adapter (Loopback Test)

Most modern set-top box mainboards operate on a strict or even 1.8V TTL logic system. If your USB-to-UART bridge (such as a CH340, CP2102, or FTDI adapter) has its physical jumper set to 5V, the chip may activate its internal over-voltage protection circuits, or worse, clip the data lines and refuse to process signals. 3. Floating Ground Reference

If the characters echo back on your screen, your adapter and PC drivers work perfectly. If nothing appears, replace your adapter or reinstall its Prolific, FTDI, or CH340 driver. 3. Master the Power-Cycle Timing

Plug the adapter into your computer and open a terminal emulator (such as PuTTY or Tera Term).

Use a high-quality USB-A to USB-A cable, ensuring it is as short as possible.

If you have executed the steps above perfectly and the error persists, you are likely dealing with deep circuit protection mechanisms or hardware degradation.

To avoid encountering such errors in the future: