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Subsystem Command
Trees
The Command Tree Structure
Most programming tasks in v olve subsystem commands. SCPI uses
a hierarchical structure for subsystem commands similar to the le
systems on most computers. In SCPI, this command structure is
called a c ommand tr e e .
Figure 1-25. A Simplified Command Tree
In the command tree sho wn in Figure 1-25, the command closest to
the top is the r o ot c ommand , or simply the r o ot . Notice that y ou
m ust follo w a particular p ath to reac h low er lev el sub commands. F or
example, if y ou wish to access the GG command, y ou m ust follow the
path AA to BB to GG .
P aths Through the Command Tree
T o access commands in dieren t paths in the command tree, y ou
m ust understand ho w an instrumen t in terprets commands. A sp ecial
part of the instrumen t rm w are, a p arser , deco des eac h message sen t
to the instrumen t. The parser breaks up the message in to comp onen t
commands using a set of rules to determine the command tree path
used. The parser k eeps track of the curr ent p ath , the lev el in the
command tree where it exp ects to nd the next command y ou send.
This is imp ortant b ecause the same k eyw ord may app ear in dieren t
paths. The particular path y ou use determines ho w the k eyw ord is
in terpreted. The following rules are used b y the parser:
Power On and R eset
After p o w er is cycled or after *RST, the curren t path is set to the
ro ot.
Message T erminators
A message terminator, suc h as a <new line > c haracter, sets the
curren t path to the ro ot. Many programming languages ha v e
output statements that send message terminators automatically .
The paragraph titled, \Details of Commands and Resp onses,"
discusses message terminators in more detail.
1-68 Getting Started Programming