Jean,
the parallel port is a long one, with about 25 holes/pins in 2 rows of 13 & 12. (Sometimes called a longer serial port, also known as DB-25, and RS-232)
The serial port is a shorter one - 9 holes or pins, in 2 rows of 5 & 4.
If there is short blue one with 3 rows, it'll be the VGA graphics port for a monitor.
The parallel port is what most older plotters used - and our Rolands all do.
You can buy a PCI card, (or for most newer computers, a PCI-E card), to slot onto the motherboard, to give you the needed expansion parallel or serial ports out the back.
Yes, you can get USB to serial converter leads, but I have never found one successful for any of our stuff.
Others have more success, but not me with USB to serial or parallel.
(serial leads send one piece of data at a time, parallels can send 2 sets of data at a time down the lead.)
You can get serial adapters to go from 9 to 25 pin, and some are bidirectional, and some are a null modem, with 2 pairs of wires crossed over, instead of being straight-through. There is a time when the null modems are needed, (eg on our cnc router), and a time when you just want one to make an extension, or to convert a female to male end etc.
I expect your plotter will have a parallel port lead at the PC end, and maybe even an old printer centronics plug at the plotter end.
Hope this helps!
