Epson 20 - Manual (Page 167)

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. Note that if you connect a printer cable instead of a loop-back
connector, you will get errors.
Alternate Parallel Port Check
Use this option to test the operation of an additional parallel port. To
perform the test, you must insert the special loop-back connector into
the alternate parallel port so that the computer can check individual
pins of the port.
This test is similar to the Parallel Port Check. For more details, see
the description of the Parallel Port (Printer Interface) Check.
Parallel Port (on Video Adapter) Check
Use this option to test the operation of an additional parallel port on
a video adapter. To perform the test, you must insert the special loop-
back connector into the parallel port on the video adapter so that the
computer can check individual pins of the port.
This test is similar to the Parallel Port Check. For more details, see
the description of the Parallel Port (Printer Interface) Check.
Serial Port (RS-232C) 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 RS-232C port so that the
computer can check individual pins of the port. Contact your dealer
if you need a loop-back connector. Note that a different connector is
required to test the parallel port.
Performing System
Diagnostics C-23