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root
level 1
BB
cc
DD
c
+I
rtl
level 2 EE FF GG
HH
Figure
l-25.
A Simplified Command Tree
JJ
In the command tree shown in Figure
l-25,
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 subcommands. For
example, if you wish to access the GG command, you must follow the
path AA to BB to GG.
Paths Through the Command Tree
To access commands in different paths in the command tree, you
must understand how an instrument interprets commands. A special
part of the instrument firmware, a purser, decodes each message sent
to the instrument. 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 where it expects to find the next command you send.
This is important because the same keyword may appear in different
paths. The particular path you use determines how the keyword is
interpreted. The following rules are used by the parser:
8 Power On and Reset
After power is cycled or after
*RST,
the current path is set to the
root.
n Message Terminators
A message terminator, such as a <new line> character, sets the
current path to the root. Many programming languages have
output statements that send message terminators automatically.
The paragraph titled,
“Details of Commands and Responses,”
discusses message terminators in more detail.
8
Colon
When it is between two command mnemonics, a colon moves the
current path down one level in the command tree. For example,
the colon in MEAS:VOLT specifies that VOLT is one level below
MEAS.
When the colon is the first character of a command, it specifies
that the next command mnemonic is a root level command. For
example, the colon in
:
INIT
specifies that
INIT
is a root level
command.
l-68
Getting Started Programming