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IEEE 488.2 Common Commands
*OPC (Operation
Complete)
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.
NOTE: The *OPC command does not necessarily cause bit 0 in the Standard Event Status Register
to be set true immediately following a measurement completion or the completion of a state
or condition change in the Test Set. 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. If an active (on) measurement displays four dashes (----) and
the Test Set is configured with a PCS Interface, the *OPC, *OPC? and *WAI commands
are never “done”. Turn off any measurements that may cause this condition, or command
the Test Set to single trigger mode. If the Test Set is not configured with a PCS Interface,
and an active measurement displays four dashes (----), the conditions required to satisfy
*OPC, *OPC? and *WAI commands may be satisfied, but a valid measurement result will
not be obtained.It is only when all active measurements are done and the one-second timer
has elapsed, that the *OPC, *OPC? and *WAI commands are satisfied. Many state changes
or measurement cycles take much less than one second. For this reason, *OPC should not
be used when program execution speed is an issue.