Epson EQUITY 3865X/20 PLUS - Manual (Page 31)

mouse connector
Caution
Although the mouse and keyboard connectors are physically
identical, they cannot be used interchangeably. Be sure to
insert the mouse plug into the mouse port.
Once you have connected a mouse, you may need to add
commands to your MS-DOS CONFIG.SYS and
AUTOEXEC.BAT files to enable your computer to use a mouse.
See your MS-DOS Reference Manual and the manual that came
with your mouse for instructions.
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Setting Up Your System