Remote Control
R&S
®
RTM20xx
259
User Manual 1317.4726.02 ─ 01
15.1.5 Command Sequence and Synchronization
IEEE 488.2 defines a distinction between overlapped and sequential commands:
●
A sequential command is one which finishes executing before the next command
starts executing. Commands that are processed quickly are usually implemented as
sequential commands.
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An overlapping command is one which does not automatically finish executing before
the next command starts executing. Usually, overlapping commands take longer to
process and allow the program to do other tasks while being executed. If overlapping
commands do have to be executed in a defined order, e.g. in order to avoid wrong
measurement results, they must be serviced sequentially. This is called synchroni-
zation between the controller and the instrument.
Setting commands within one command line, even though they may be implemented as
sequential commands, are not necessarily serviced in the order in which they have been
received. In order to make sure that commands are actually carried out in a certain order,
each command must be sent in a separate command line.
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 might defer executing the individual commands until a program message terminator
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.
Basics
