IBM Personal Computer AT Manual To Operations page 264

Example 6
Screen Response
Audio Response
Action
+
See below
Blank screen,
No response
=)
Blinking
or
cursor or screen
Any response
may display
anything
Refer to the display adapter installation instructions in
your Installation and Setup manual. After setting your
display's switch, return to "Checking Electrical
Connections" in this section.
If your switch is set correctly, have your system unit
serviced. Be sure your Key Lock key is with your
system unit.
2-20 Error Examples