B&K Precision 4079 - Manual - Manual (Page 49)

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parser how many 8-bit bytes are being sent.
Indefinite Form
The Indefinite Form has the structure
- # - 0 – 8-bit byte – LF^EOI
Some Program Message Units either require, or can accept, more than one data element. Program data elements are separated
from each other by the Program Data Separator. It is defined as optional whitespace characters followed by a comma (','),
which in turn is followed by optional whitespace characters.
There are two types of Program Message Units: Command Message Units and Query Message Units. A Query differs from a
Command in that the Program Header is terminated with a question mark ('?'). For example, the frequency might be queried
with the following query:
FREQ?
Some Query Message Units accept data, giving the device more specific information about what is being queried. In many
cases the Query Message Unit may optionally be supplied with the MIN or MAX mnemonics as data. This tells the device to
return the minimum or maximum value to which the parameter may currently be set. For example,
FREQ? MAX
will return the maximum value to which the frequency may currently be set.
Not all Program Message units have query forms ( for example, STATUS:PRESET), and some Program Message Units
might have only the query form (for example SYSTEM:VERSION?).
The instrument puts the response to the query into the output queue, from where it may be read by the controller. The Status
Byte MAV bit is set to indicate to the controller that a response is ready to be read.
4.10.2 SCPI Command Structure
SCPI commands are based on a hierarchical structure. This allows the same instrument-control header to be used several
times for different purposes, providing that the mnemonic occurs in a unique position in the hierarchy. Each level in the
hierarchy is defined as a node. Mnemonics in the different levels are separated from each other by a colon (':'). The first
Program Message Unit, or command, in a Program Message is always referenced to the root node. Subsequent commands are
referenced to the same level as the previous command. A Program Message Unit having a colon as its first character causes
the reference to return to the root. This process is defined by IEEE 488.1, section A.1.1. Consider the following examples:
a) The following command may be used to set the amplitude and the offset of the signal.
SOURCE:VOLTAGE:AMPLITUDE 5V;OFFSET 2V
Note that the offset command is referenced to the command preceding it: the OFFSET mnemonic resides at the same node as
the AMPLITUDE command.
b) This command set the frequency and the amplitude.
SOURCE:FREQUENCY 2KHZ;VOLTAGE:AMPLITUDE 4V
The FREQUENCY and VOLTAGE mnemonics are at the same level.
c) When Program Message Units describe different subsystems, a colon prefix must be used to reset the command
reference to the root. Here the frequency and the output state are set.