yes & it's near the same as the "export HPGL" format...

I found something : the extension menu uses the "plotter.py" code, that add an extra "PU,0,0;IN" code. If you remove that, I think it will work. I had to test next week (my plotter is at my fablab ...)

Hello, I checked the code sent by Inkscape to the Plotter via a virtualSerialPort & the HPGL code exported (and sent manually via a cat command).
They are exactly the same.
So, the problem should come from pySerial implementation ... Investiagation continues.

Hello, I have exactly the same problem, and I started to read the code of the extension, modifiying the HPGL end code... Which doesn't work. One surprising thing : when I show the "debug code", I copy/paste the HPGL output & send it directly to the plotter, there is NO more going-back-...