Star LC-8521 User Manual page 27

Connecting to your computer with a serial cable 19
Plug one end of the serial cable into the serial port of your computer. The
parallel port should be labeled "COM1," "COM2," or something similar.
Lift up the back of the printer and insert two books or other similar objects
underneath it to keep the printer raised.
Plug the other end of the serial cable into the socket on the back of the
printer and secure it in place with the screws.