IBM Personal Computer AT Manual To Operations page 327

7
Did you receive a 6XX error?
XX:XX:XX
ERROR
SYSTEM UNIT 6XX
END - TEST 600
PRESS "ENTER" TO CONTINUE
?
(X
can be any number.)
YES Note the error and have your system unit
serviced. Be sure your Key Lock key is
with your system unit.
NO Go to Step 8.
Test 600-7