HP 7470A Interfacing And Programming Manual page 148

Operation
While operating in this environment, the plotter may be in one of three
states: on—line,programmed-off; on—line,programmed-on; or monitor
mode.
On-Line, Programmed-Off State
The plotter can only be in this state if the Y/Dswitch on the rear panel
is set to Y(used with Y-cable). When this switch is set to Y,the plotter is
placed in the on—line, p rogrammed-off state by either turning the
plotter's LINE switch to ONor by receipt of a plotter off instruction from
the computer or of a terminal—generatedBreak signal while the plotter
is in the on-line, programmed-on state. In the on—line, programmed-off
state, the plotter's processor passes data between the computer and the
terminal as shown in the following diagram. The plotter will respond
only to a plotter on instruction from the host computer. "
COMPUTER Y—CABLE
' CONNECTOR
TERMINAL
PROCESSOR
SCANS FOR
"PLOTTER ON"
INSTRUCTION
PLOTTER
Plotter in On-Line, Programmed-Off State
10-6 RS-232-C/CCITT V.24INTERFACING