Look under File > Plot Setup, there is a driver for the HP Designjet 430. I had a HP 650C (since gone to plotter heaven) and had added a Directjet card which added an ethernet connection to the plotter. I don't know about a usb-serial pin connection, but the Directjet card made a much faster connection. You could also try saving plot (.plt) files (File > Plot...), then transferring them to your old computer that is connected to the plotter.
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