Programming instructions

Command Reference
SCPI Basics
Chapter 4 143
Command Tree
Most programming tasks involve subsystem commands. SCPI uses a structure for subsystem
commands similar to the file systems on most computers. In SCPI, this command structure is
called a command tree and is shown in Figure 4-2.
Figure 4-2 Simplified Command Tree
The command closest to the top is the root command, or simply “the root.” Notice that you
must follow a particular path to reach lower level commands. In the following example,
:POWer represents AA, :ALC represents BB, :SOURce represents GG. The complete command
path is :POWer:ALC:SOURce? (:AA:BB:GG).
Paths Through the Command Tree
To access commands from different paths in the command tree, you must understand how the
signal generator interprets commands. The parser, a part of the signal generator firmware,
decodes each message sent to the signal generator. The parser breaks up the message into
component commands using a set of rules to determine the command tree path used. The
parser keeps track of the current path (the level in the command tree) and where it expects to
find the next command statement. This is important because the same keyword may appear
in different paths. The particular path is determined by the keyword(s) in the command
statement.
A message terminator, such as a <new line> character, sets the current path to the root.
Many programming languages have output statements that automatically send message
terminators.
NOTE The current path is set to the root after the line-power is cycled or when
*RST is sent.