IBM 3270 Maintenance Manual page 406

Undetermined Problems
1. Set the Power switch on the system unit (and
expansion unit, if attached) to Off.
2. Unplug the system unit power cord from the
power outlet.
3. Remove the keyboard adapter cable from the
system unit (0301 and 0302 errors may
appear).
CAUTION
Do not connect or disconnect a coaxial
cable during an electrical storm.
4. Remove any external devices except the
display that are attached to the system unit.
CONTINUE
Display
Signal
Cable
October 1984 3-9999-1