Hummmm COM7? Normal setups don't have a COM 7. First 4 com ports are directly addressable from a PC. Higher numbers need a special device driver. You may have more luck with a PCI expansion card (you can get a parallel and one serial card for about 15.00). See if the specific driver for the USB will let you change the serial port to COM 1 and there is anything about "legacy" drives try that selection.
It may also need a special serial cable. If you are not using a cable that came with the unit (from Roland) you may need to buy a true "serial" cable (VS an "extension" serial cable). The former has pins 2 & 3 crossed and 4 & 5 crossed. At that pint the rev and transmit will talk to each other. Next the settings in Windows for the COM port HAVE to be what the printer calls for. If the BAUD rate, parity or word size do not match it will just sit there and stare at you. Your printer setup in Windows will have to match the port (com7 in your case).
I remember the early days of PC's and printers and having to hook up serial printers and external modems. It was a painful process even when we had the proper device driver and the printer manual. We carried a serial port breakout with status LED's and jumper wires so we could see everything but the baud rate. We would try the normal combinations until it worked then make a cable that had the right pins connected (or cross connected). Most PC serial connectors are 9 pin. Most plotter serial connectors are 25 pin. A simple 9 to 25 pin adapter may or may not work.
See if you can get a manual. It should give you the pin out of the serial connector on the plotter.
Even if you get the communications working:
Later Windows (than 98) don't include an HPGL driver and the older version won't work. You will have to get an updated driver from Roland (if they have one). I'm not sure they used standard HPGL commands (they would have to pay a royality to HP) so they did variations on the HPGL command set or signaling. Because of that the generic programs won't work.
One way around the mess is to use a sign program that has a driver for your specific device. I have a 24" vinyl cutter with a parallel and serial. I downloaded a Windows 2000 HPGL driver for my specific machine. I could never get it to work from Windows on any of my software. I started with serial but gave up on that early on (although it may work with the proper program as an output) and I know what I am doing!
I finally bought and older copy of SignLab that had the drivers listed for my specific machine and PRESTO, it works (although I have to cut using the SignLab program. I still do all my layout and design in CorelDraw and just import and cut via SignLab.
Good luck on your quest. Getting older equipment to work with modern computers can be a challenge. You may start to understand whay you got a good deal on the machine (:-)