Remote Control
R&S
®
ZNC
348
User Manual 1173.9557.02 ─ 13
5.4.5 Output Unit
The output unit collects the information requested by the controller, which it receives from
the data set management. It processes it according to the SCPI rules and makes it avail-
able in the output buffer. If the information requested is longer, it is made available in
portions without this being recognized by the controller.
If the instrument is addressed as a talker without the output buffer containing data or
awaiting data from the data set management, the output unit sends the error message
Query UNTERMINATED
to the status reporting system. No data is sent on the GPIB bus
or via the Ethernet, the controller waits until it has reached its time limit. This behavior is
specified by SCPI.
5.4.6 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 synchroni-
zation between the controller and the analyzer.
According to
chapter 5.4.3, "Data Base and Instrument Hardware"
commands within one command line, even though they may be implemented as sequen-
tial 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
Command Processing