Epson 10020 - Manual (Page 188)

When you continue the test, the computer checks that the
printer is responding correctly. This test detects whether the
printer is off-line or whether an interface error exists. If no
errors occur, the computer sends a repeating sequence of
characters and bit-image data to the printer until you press any
key. The pattern looks like this:
The text data includes all the characters commonly used by
programs that require foreign languages or graphic characters. If
your printer prints different characters than you see in the
illustration, some printed characters may not be what you
expect with certain software.
The bit-image data is sent to the printer using a command
(ESC K) compatible with Epson and IBM printers. If this
pattern is printed correctly, you can use the MS-DOS
GRAPHICS program to print out copies of graphics screens.
Note
Even if the test runs only for a short time, your printer may
store many characters in its buffer. To stop printing, set the
printer off-line.
Hard Disk Drives and Controller Check
Use this option to test the performance of the hard disk drive
installed in your computer. If any errors occur, have the drive
checked and serviced by your Customer Care Center.
Performing System Diagnostics D-31