IBM 3278 Technical Reference page 16

Bit 0
This bit is set by the IBM Personal Computer,
causing the emulation adapter to become enabled and
on-line. After this bit is set, the first POLL causes a
POR response to be sent to the control unit. The bit
is initially reset during hardware power-on. It is also
reset by the RESET command or by the IBM Personal
Computer. When this bit is reset, the emulation
adapter ignores all transmissions from the control
unit. Thus, it causes the control unit to timeout and
set the "GO TO SLEEP" latch for that line.
Bit 1 Reserved.
Bit 2 Reserved.
Bit 3 This bit is set by the IBM Personal Computer and
notifies the emulation adapter that the Scan Code
Register has been loaded. This data is then handled as
a keystroke. This bit is reset by a POLL ACK or
RESET command from the control unit.
Bit 4 Reserved.
Bit 5 This bit is set by the IBM Personal Computer. It
allows the IBM Personal Computer to do limited
functional testing of the IBM Personal Computer
3278/79 Emulation Adapter card. The coaxial link is
automatically disabled.
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