Agilent Technologies 669xA 2. Partial Command Tree Active Header Path, Moving Among Subsystems

Figure 2-2. Partial Command Tree

Active Header Path

In order to properly traverse the command tree, you must understand the concept of the active header path. When the power supply is turned on (or under any of the other conditions listed above), the active path is at the root. That means the interface is ready to accept any command at the root level, such as TRIGger or STATus in Figure 2-2. Note that you do not have to precede either command with a colon; there is an implied colon in front of every root-level command.

If you enter STATUS, the active header path moves one colon to the right. The interface is now ready to accept : OPERATION, :PRESET, or QUESTIONABLE as the next header. Note that you must include the colon, because it is required between headers.

If you next enter :OPERATION, the active path again moves one colon to the right. The interface is now ready to accept :EVENT?, CONDITON?, ENABLE, NTRANSITION, or PTRANSITION as the next header.

If you now enter :ENABLE, you have reached the end of the command string. The active header path remains at :ENABLE. If you wished, you could have entered :ENABLE 18;PTRANSITION 18 and it would be accepted. The entire message would be STATUS:OPERATION:ENABLE 18;PTRANSITION 18. The message terminator after PTRANSITION 18 returns the path to the root.

The Effect of Optional Headers

If a command includes optional headers, the interface assumes they are there. For example, if you enter STATUS: OPERATION?, the interface recognizes it as STATUS: OPERATION: EVENT? (see Figure 2-2). This returns the active path to the root (:STATUS). But if you enter STATUS: OPERATION: EVENT?, then the active path remains at :EVENT. This allows you to send STATUS: OPERATION: EVENT?; CONDITION? in one message. If you tried to send STATUS:OPERATION?;CONDITION? the command path would send STATUS:OPERATION:EVENT? and then return to :STATUS instead of to :CONDITION.

The optional header SOURCE precedes the current, digital, and voltage subsystems (see Figure 3-2). This effectively makes :CURRENT, :DIGITAL, and :VOLTAGE root-level commands.

Moving Among Subsystems

In order to combine commands from different subsystems, you need to be able to restore the active path to the root. You do this with the root specifier (:). For example, you could clear the output protection and check the status of the Operation Condition register as follows (see Figure 3-2):

OUTPUT:PROTECTION:CLEAR

STATUS:OPERATION:CONDITION?

By using the root specifier, you could do the same thing in one message:

OUTPUT:PROTECTION:CLEAR;:STATUS:OPERATION:CONDITION?

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