Preventing Overlapping Execution - R&S SMB100A Operating Manual

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R&S SMB100A
Example: Commands and queries in one message
The response to a query combined in a program message with commands that affect
the queried value is not predictable.
The following commands always return the specified result:
:FREQ:STAR 1GHZ;SPAN 100 :FREQ:STAR?
Result:
1000000000 (1 GHz)
Whereas the result for the following commands is not specified by SCPI:
:FREQ:STAR 1GHz;STAR?;SPAN 1000000
The result could be the value of STARt before the command was sent since the instru-
ment can defer executing the individual commands until a program message termina-
tor is received. The result could also be 1 GHz if the instrument executes commands
as they are received.
As a general rule, send commands and queries in different program messages.
Example: Overlapping command with *OPC
The instrument implements INITiate[:IMMediate] as an overlapped command.
Assuming that INITiate[:IMMediate] takes longer to execute than *OPC, sending
the following command sequence results in initiating a sweep and, after some time,
setting the OPC bit in the ESR:
INIT; *OPC.
Sending the following commands still initiates a sweep:
INIT; *OPC; *CLS
However, since the operation is still pending when the instrument executes *CLS, forc-
ing it into the "Operation Complete Command Idle" state (OCIS), *OPC is effectively
skipped. The OPC bit is not set until the instrument executes another *OPC command.

6.4.1 Preventing Overlapping Execution

To prevent an overlapping execution of commands, one of the commands *OPC,
*OPC? or *WAI can be used. All three commands cause a certain action only to be
carried out after the hardware has been set. By suitable programming, the controller
can be forced to wait for the corresponding action to occur.
Operating Manual 1407.0806.32 ─ 21
Remote Control Basics
Command Sequence and Synchronization
267