Sync Search; Selected; Transmit; Status - IBM 3270 Operator's Manual

SYNC SEARCH tells you that your display station is available if
the computer should want to select it for communication. If a
display station is selected and is in the process of displaying
information sent to it by the computer, it cannot be selected for
further communication until the first task is completed.
SELECTED is on while your display station is selected by, and is
communicating with, the computer.
TRANSMIT comes on when your display station is sending data
over telephone lines to the computer.
Your display station must tell the computer when it detects an
error; it must inform the computer of its status. The STATUS
indicator is on when an equipment problem exists in the display
station which is not caused by the operator.
Operating the 3270
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SYNC SEARCH Indicator
SELECTED Indicator
TRANSMIT Indicator
STATUS Indicator