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In most case, you can not connect a wire to the PCB board on a finished product to get the operation log or debug.

This design allows you to obtain device information through a led, you only need to connect led to the printing device serial port.

This design tested with 9600 baud rate.

Step 1:

Connect a led to the serial port TX.

Step 2:

Build a amplifier board with a photo detector.

Step 3:

Connect the output to a usb-serial adapter or a oscilloscope,

and place the detector close to the debug led.

it works.

Nice!!

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