IBM Personal Computer Manual To Operations page 121

Test 400
1
Do you have an IBM Monochrome Display and
Printer Adapter installed in your system?
NO - Continue with the next Test.
YES - Did the following appear on your
screen?
XX:XX:XX
ERROR SYSTEM UNIT 4XX
NO - Continue with the next step.
YES - Record the error message and have the
unit that contains the adapter serviced.
April 1984 Test 400-1