IBM 3270 Maintenance Manual page 340

CAUTION
Do not connect or disconnect the coaxial
cable during an electrical storm.
1. Disconnect and reconnect the coaxial cable to
your system unit.
2. Press "Y," then Enter.
DID "CONNECTED TO CONTROL UNIT"
APPEAR ON YOUR SCREEN?
YES Your problem is resolved; it was most
likely a poor connection. To exit the test,
press N, then Enter.
NO
October 1984 3-2800-5