HP 8921A Programmer's Manual page 232

Triggering Measurements
Trigger Commands
232
:TRIGger:IMMediate
The :TRIGger:IMMediate command tells the Test Set to "start a measurement
cycle now." The type of triggering used depends on the trigger mode settings.
This command is equivalent to a Group Execute Trigger Command (GET) as
defined by IEEE 488.1-1987 or a Trigger Common Command (*TRG) as defined
by IEEE 488.2-1987. The IMMediate statement is implied and is optional.
Syntax
:TRIGger:IMMediate
Example
OUTPUT 714;"TRIG:IMM"
or
OUTPUT 714;"TRIG"
:ABORt
The :ABORt command tells the Test Set to stop a currently executing
measurement cycle and get ready for a new HP-IB command. If for any reason a
valid measurement cannot be made, this command allows the control program to
terminate the requested measurement and regain control of the Test Set.
Syntax
:TRIGger:ABORt
Example
OUTPUT 714;"TRIG:ABOR"