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Programming
Reviewing SCPI Syntax Rules
Program message terminator
The string of instructions sent to the instrument are executed after the
instruction terminator is received. The terminator may be either a new-line
(NL) character, the End-Or-Identify (EOI) line asserted, or a combination of
the two. All three ways are equivalent. Asserting the EOI sets the EOI control
line low on the last byte of the data message. The NL character is an ASCII
linefeed (decimal 10). The NL terminator has the same function as an EOS
(End Of String) and EOT (End Of Text) terminator.
Querying data
Data is requested from the instrument using a query. Queries can be used to
find out how the instrument is currently configured. They are also used to get
results of measurements made by the instrument, with the query actually acti-
vating the measurement. String responses are returned as upper-case letters.
Queries usually take the form of a command followed by a question mark (?).
After receiving a query, the instrument places the answer in its output queue.
The answer remains in the output queue until it is read or another command is
issued. For example, the query
OUTPUT 720;”:CALCULATE2:POINTS?”
places the number of points in the data set in the output queue. In HP BASIC,
the controller input statement:
ENTER 720;Range
passes the value across the bus to the controller and places it in the variable
Range. A newline character is appended to the response.
Sending another command or query before reading the result of a query
causes the output queue to be cleared and the current response to be lost.
This also generates an error in the error queue.
The output of the instrument may be numeric or character data depending on
what is queried. Refer to the specific commands for the formats and types of
data returned from queries.
You can send multiple queries to the instrument within a single program mes-
sage, but you must also read them back within a single program message. This
can be accomplished by either reading them back into a string variable or into
multiple numeric variables. When you read the result of multiple queries into
string variables, each response is separated by a semicolon.