R&S ZVL
Remote Control
Command Processing
Operating Manual 1303.6580.32-05
332
Command Sequence and Command 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 analyzer.
According to section
Data Set and Instrument Hardware,
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.
Examples:
Example 1: 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. Sending
:FREQ:STAR 1GHZ;SPAN 100
:FREQ:STAR?
always returns 1000000000 (1 GHz). When:
:FREQ:STAR 1GHz;STAR?;SPAN 1000000
is sent, however, the result is not specified by SCPI. The result could be the value of
STARt
before the
command was sent since the instrument 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.
Example 2: Overlapping command with *OPC
The analyzer implements
INITiate[:IMMediate]
as an overlapped command. Assuming that
INITiate[:IMMediate]
takes longer to execute than *
OPC
, sending the command sequence
INIT; *OPC.
results in initiating a sweep and, after some time, setting the OPC bit in the ESR. Sending the commands:
INIT; *OPC; *CLS
still initiates a sweep. Since the operation is still pending when the analyzer executes
*CLS
, forcing it into
the Operation Complete Command Idle State (OCIS), *OPC is effectively skipped. The OPC bit is not set
until the analyzer executes another *OPC command.
The analyzer provides only one overlapped command,
INITiate<Ch>[:IMMediate]
.
What is said below is not relevant for the other (sequential) SCPI commands.
