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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.

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Agilent Technologies PSG Manual de Instruções Página 157