Epson TM-T20 Technical Reference Manual page 38

Connecting the serial interface (RS-232) cable
Be sure to turn off the power supply for both the printer and host computer before
connecting the cables.
WARNING
1
Insert the serial cable connector firmly into the interface connector.
When using connectors equipped with screws, tighten them to secure the connectors
firmly.
When using interface cables equipped with a grounding line, attach the ground line to the
screw hole marked "FG" on the printer.
2
Connect the other end of the serial cable to the host computer.
38
Serial cable