3.2.6 Command String Format
Control of the PPM is accomplished by using four-character mnemonic commands. Some of these require
one or more numerical entries (arguments). The command structure is intended to be taken literally; there
are no optional formats. Thus, if two arguments are specified for a command, both must be given for the
command to be processed.
A character string is terminated with a line feed character and/or an EOI. Carriage returns are ignored.
Therefore, a carriage return/line feed sequence will act as a string terminator and will be treated as a line
feed. More than one command (and respective arguments) can be sent in a command string as long as the
string does not exceed 128 characters, including the terminator. If an EOI is sent and the last character is
not a line feed, only 127 characters can be sent. None of the commands can be processed until the string
is terminated, and then they will be processed in the order they were given (see Output Modes in
Section 3.2.5 regarding conflicting output commands).
When the command string is received, it is first checked for syntax and numerical values, and commands
are reviewed to make sure they are valid. If a syntax error or an invalid data error is detected, or the
execution of the command string would lead to an operation error, then none of the commands will be
executed and a service request will be generated if SRQ is enabled.
A very forgiving syntax is allowed since the PPM can distinguish between any command and any
argument, with no separator character required between commands or a command and its argument. The
only time a separator character is required is between arguments when the command has two or more
arguments. A separator is defined to be a space, a comma, or a semi-colon. Any number of separators are
allowed before the first command, between a command and an argument, between arguments, between
the last argument of a command and the next command, or at the end of the string without effecting the
PPM’s interpretation of the command string. Commands can be in upper or lower case format.
An argument consists of a digit string which can optionally be preceded by a positive (+) or negative (-)
sign. If no sign is present, the number is assumed to be positive. The digit string can also contain a
decimal point if desired. The dimension of each argument is fixed, therefore no dimensional identifier is
needed or allowed, only a number. This will be described in the command description to follow. 1018B
command arguments follow a different format as is discussed in Section 3.5.
3.2.7 Power Measurement Data Output Format
The measurement data output formats transmitted by the PPM over the GPIB depend on whether the PPM
is in the dBm or W power mode of operation. These modes are entered by using GPIB commands as
described in Section 3.3.3 or, when the PPM is not in the Remote mode, by pressing [dB/mW] on the
front panel.
Power measurements over the GPIB are taken only during the Update Trigger Reset or Update Data
Continuously output modes (see Section 3.2.5).
To initiate a power measurement over the GPIB, a command is sent to the PPM from the controller and
then talk-addressed. When the PPM is triggered, it will transmit its data through the GPIB to the
controller. See the following command descriptions for the commands required to take data. The
measurement data strings are discussed in subsequent sections.
Log (dBm) Data Format
When the PPM is in the dBm mode (as indicated in the display), the first three letters of the data string
are DBM. This indicates that the data represents a dBm value. The following letter indicates which
detector (channel A or B) was taking the measurement. This is followed by a polarity sign, and then a
string of five digits with a decimal point after the third digit.
Each measurement taken in the Update Trigger Reset Mode will be terminated by a carriage return
followed by a line feed sent with EOI. Measurements taken in the Update Trigger Continuously Mode
will be separated with commas. Examples are:
Series 8500A Peak Power Meters
3-12
Manual No. 20790, Rev C, November 1998
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