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5-46. Definition: Queries and Commands
Messages directed to the 5790A fall naturally into two categories: commands and
queries. Commands instruct the 5790A to do something or to set a value; no response is
expected. Queries generally ask only for information from the 5790A; a response is
always expected. Some queries also require the 5790A to take action. For example, the
*TST? query has the 5790A do a self test, then send the result to the controller. A query
always ends with a question mark. A command never ends with a question mark. (In the
alphabetical listing of commands in Section 6, all commands and queries are mixed
together and called commands.)
Most query responses are generated instantly on receipt of the query, i.e., queries
generate their output when the 5790A executes the query rather than when the controller
attempts to read the response. The 5790A simply generates the requested message and
places it in an area of memory called the output buffer. When the controller addresses the
5790A as a talker, the contents of the output buffer are transmitted to the controller.
Some messages have both query and command forms (e.g., *PUD and *PUD?). In such
cases, the command generally sets the value of a parameter and the query generally
returns the most recent value of the parameter. Some messages are queries only (e.g.,
*IDN?). Some messages are commands only (e.g., *RST).
5-47. Functional Elements of Commands
Table 5-6 lists the functional elements of commands described by the IEEE-488.2
standard that are used by the 5790A. This table is for those who have a copy of the
standard and want to use it to pursue additional information. The standard provides full
definitions and syntax diagrams for each element.
5-48. Interface Messages
Interface messages manage traffic on the bus. Device addressing and clearing, data
handshaking, and commands to place status bytes on the bus are all directed by interface
messages. Some of the interface messages occur as state transitions of dedicated control
lines. The rest of the interface messages are sent over the data lines with the ATN signal
true. (All device-dependent and common commands are sent over the data lines with the
ATN signal false.)
IEEE-488 standards define interface messages. Table 5-7 lists the interface messages that
the 5790A accepts. Table 5-7 also shows the BASIC statement you execute on the 1722A
Controller to generate the interface message. Table 5-8 lists the interface messages that
the 5790A sends. The mnemonics listed in the tables are not sent in BASIC PRINT
statements as commands are; in this way they are different from device-dependent and
common commands.
Interface messages are handled automatically in most cases. For example, handshake
messages DAV, DAC, and RFD automatically occur under the direction of an
instrument's interface itself as each byte is sent over the bus.
5-49. Remote Program Examples
The following programming examples illustrate ways to handle errors, to take
measurements, take a number of successive readings, lock the range, and calibrate the
5790A. These excerpts from programs are written in Fluke BASIC for the Fluke 1722A
Instrument Controller.
Remote Operation
Remote Program Examples
5
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