Network and Remote Operation
R&S
®
FSPN
261
User Manual 1179.4363.02 ─ 01
10.1.6
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.
●
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 synchronization 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 cer-
tain 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 termi-
nator 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.
Remote Control Basics
