User`s manual

96 Chapter 4
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IEEE Common Commands
Common Command Descriptions
*OPC? (Operation Complete Query)
The *OPC? query allows for synchronization between the Test Set and
an external controller by reading the Output Queue or by polling the
Message Available (MAV) bit in the Status Byte Register. The *OPC?
query causes the Test Set to place an ASCII character, 1, into its Output
Queue when the Test Set completes all pending operations. A
consequence of this action is that the MAV bit in the Status Byte
Register is set to the 1 state.
NOTE 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.
Using the *OPC? query by reading Output Queue
Bit 4 in the Service Request Enable Register is set to a value of zero
(disabled). The *OPC? query is sent to the Test Set at the end of a
command message data stream. The application program then
attempts to read the *OPC? query response from the Test Set’s Output
Queue. The Test Set will not put a response to the *OPC? query into the
Output Queue until the commands have all finished.
NOTE Reading the response to the *OPC? query has the penalty that both the
HP-IB bus and the Active Controller handshake are in temporary
holdoff state while the Active Controller waits to read the *OPC? query
response from the Test Set.