Here are some random thoughts which may or may not be of use. I'll consider the serial option.
There are 2 or 3 pieces to this puzzle, the drafting software that sends the data to the com port, the usb-rs-232 converter and maybe the 9-pin to 25-pin cable if your plotter has a 25-pin connector.
I used to use HP plotters years ago so I'm assuming that the Roland either uses the HPGL (extension either .plt or .hgl) format or something similar.
Could you create a virtual serial port so that your drafting software could send its plot to a file to check that part is working OK?
Having such a file would be useful for testing the other bits by typing something like "copy testplot.plt com7:" at a command prompt (com port number can be checked in device manager)
In terms of the converter, make sure that the one you have has hardware flow control. Plotters are slow so if the laptop spews all of the data at the plotter in one continuous stream it won't be able to keep up.
It needs the RTS and CTS lines to regulate the flow. (there are other ways such as xon-xoff but hardware flow control was typical on the HP plotters)
You could check that the signal physically appears on the TX data line either with an oscilloscope or a second laptop with a serial port running a terminal program and connected with a null modem cable.
Don't under estimate the last part if its a 25-pin connector. The HP plotter we had was notoriosly picky.
Look at the replies here to get a suggestion of how it should be wired (I have not tested this, I only just found it in a search)
https://forum.arduino.cc/t/how-to-co...arduino/358142
Good luck, I always found it mesmorising to watch and listen to a plottor make an image appear.