Epson LT-286 - Manual (Page 147)

Insert the loop-back connector. Then press Y and Enter to start
the check.
The computer checks the port by writing and reading data and
control information, and reports errors for any pins that are faulty.
Serial Port (RS-232C Port) Check
Use this option to test the functions of the primary serial
communications (RS-232C) port.
To perform the test, you must insert a special loop-back connector
into the serial port so that the computer can check the individual
pins of the port. Contact your dealer if you need a loop-back
connector. Note that a different loop-back connector is required to
test the parallel port.
When you select option 11 from the DEVICE LIST menu, you see
this message:
SERIAL PORT CHECK
Attach loop-back connector to serial
port. Enter Y to start this check when
connector is attached, or Enter N to
return to the menu.
Insert the loop-back connector. Then press Y and Enter to start
the check.
First, the computer checks the serial port control lines to see that
they are able to change from high to low and vice versa. No
messages are displayed during this part of the test unless an error
occurs.
D-22 Performing System Diagnostics