Years ago, software applications could have direct access to the hardware interrupts and could access the pins of the parallel port directly. As time went by, things became more layered and permissions orientated. If your application does not work with the driver for your usb-parallel adapter, it may be impossible to solve without a custom driver. Serial via USB tends to have a lot more support these days as there are still plenty of sensors out there using RS-232. I also think that its less likely that there is any kind of custom signalling going on as compared to direct access to all of the pins of the parallel port. The most common settings for a serial port were 9600Baud, 8 data bits, no parity and 1 stop bit. Your plotter may have small dip switches to set this but if it is fixed, you need to set the laptop settings to whatever the plotter is using.