HP 8920A Programmer's Manual page 220

Common Command Descriptions
*OPC (Operation Complete)
NOTE:
CAUTION:
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The *OPC command allows for synchronization between the Test Set and an
external controller. The *OPC command causes the Test Set to set bit 0, Operation
Complete, in the Standard Event Status Register to the TRUE, logic 1, state when
the Test Set completes all pending operations. Detection of the Operation
Complete message can be accomplished by continuous polling of the Standard
Event Status Register using the *ESR? common query command. However, using
a service request eliminates the need to poll the Standard Event Status Register
thereby freeing the controller to do other useful work.
The Test Set contains signal generation and signal measurement instrumentation. The
instrument control processor is able to query the signal measurement instrumentation to
determine if a measurement cycle has completed. However, the instrument control
processor is not able to query the signal generation instrumentation to determine if the
signal(s) have settled. In order to ensure that all signals have settled to proper values, the
instrument control processor initiates a one-second delay upon receipt of the *OPC, *OPC?
and *WAI commands. In parallel with the one-second timer the instrument control
processor commands all active measurements to tell it when the measurement(s) are done.
When all active measurements are done and the one-second timer has elapsed, the *OPC,
*OPC? and *WAI commands are satisfied.
If the *OPC, *OPC? or *WAI common commands are used to determine when a
measurement has completed and the measurement is either in the OFF State or unavailable
(four dashed lines on CRT display "- - - -" ), the *OPC, *OPC? or *WAI command will
never complete.
The *OPC command should not be used for determining if a Call Processing Subsystem
state command has completed successfully. Call Processing Subsystem states do not
complete, a state is either active or not active. Using the *OPC command with a Call
Processing Subsystem state command results in a deadlock condition. The control program
will continuously query the Standard Event Status Register for the Operation Complete bit
to be set but the bit will never be set because the command never 'completes'.
For example, the following command sequence should not be used:
OUTPUT 714;"CALLP:ACTive;*OPC"
The *OPC command should not be used with any of the following Call Processing
Subsystem commands: :ACTive, :REGister, :PAGE, :HANDoff, :RELease.
The Call Processing Subsystem Status Register Group should be used to control program
flow. Refer to chapter 8, "Programming The Call Processing Subsystem"
information on controlling program flow using the Call Processing Subsystem Status
Register Group.
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