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to be used several times for different purposes, providing that the mnemonic occurs in a unique
position in the hierarchy. Each level in the hierarchy is defined as a node. Mnemonics in the different
levels are separated from each other by a colon (':'). The first Program Message Unit, or command, in a
Program Message is always referenced to the root node. Subsequent commands are referenced to the
same level as the previous command. A Program Message Unit having a colon as its first character
causes the reference to return to the root. This process is defined by IEEE 488.1, section A.1.1. Consider
the following examples:
a) The following command may be used to set the amplitude and the offset of the signal.
SOURCE:VOLTAGE:AMPLITUDE 5V;OFFSET 2V
Note that the offset command is referenced to the command preceding it: the OFFSET mnemonic
resides at the same node as the AMPLITUDE command.
b) This command set the frequency and the amplitude.
SOURCE:FREQUENCY 2KHZ;VOLTAGE:AMPLITUDE 4V
The FREQUENCY and VOLTAGE mnemonics are at the same level.
c) When Program Message Units describe different subsystems, a colon prefix must be used to reset
the command reference to the root. Here the frequency and the output state are set.
SOURCE:FREQUENCY 3KHZ;:OUTPUT:STATE ON
Common Commands may be inserted in the Program Message without affecting the instrument-control
command reference. For example,
SOURCE:VOLTAGE:AMPLITUDE 4V;*ESE 255;OFFSET 2V
In an instrument with multiple channels, the selection of which channel to use is achieved through the
use of a numeric suffix indicating the channel, attached to the root level mnemonic. Four root level
mnemonics are channel - dependent, and these are SOURce, TRIGger, OUTPut and ARBitrary. When the
channel is not specified, channel 1 is assumed. Program message units that are referred back to a
specific root level mnemonic operate on the channel specified in that mnemonic.
Examples:
a) SOUR:FREQ 5KHZ;VOLT:AMPL 3V
Sets the frequency and amplitude of the first channel (default).
b) SOUR2:FREQ 5KHZ;VOLT:AMPL 3V
Sets the frequency and amplitude of channel 2.
c) ARB2:START 100;LENGTH 50
Sets the start address and length of the waveform being output on channel 2.
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