IBM 3270 Maintenance Manual page 188

Compare each character on your screen with the
screen below, and verify that all characters are
correct.
Note: The scan code may differ on your screen
depending on which key was pressed last.
DID EACH BLOCK CHANGE TO THE
CORRECT CHARACTER?
NO
This problem could be caused by a fault in
the adapter for your display. Go run
diagnostic for your display. If the "PC
Character Sets" are displayed without
errors, replace the Keyboard/Timer
adapter. If there are errors in the PC
character sets, replace the adapter for
your display.
YES
October 1984 3-300-5