SCPI Command Completion, Opc? | Agilent Technologies 665xA, 664xA

SCPI Command Completion

SCPI commands sent to the power supply are processed either sequentially or in parallel. Sequential commands finish execution before a subsequent command begins. A parallel command can begin execution while a preexisting command is still executing (overlapping commands). Commands that affect trigger actions are among the parallel commands.

The *WAI, *OPC, and *OPC? common commands provide different ways of indicating when all transmitted commands, including any parallel ones, have completed their operations. The syntax and parameters for these commands are described in "Chapter 3 - Language Dictionary". Some practical considerations for using these commands are as follows:

*WAI	This prevents the power supply from processing subsequent commands until all pending operations
	are completed. If something prevents completion of an existing operation, *WAI can place the power
	supply and the controller in a "hang- up" condition.
*OPC?	This places a 1 in the Output Queue when all pending operations have completed. Because it
	requires your program to read the returned value from the queue before executing the next program
	statement, *OPC? could prevent subsequent commands from being executed.
*OPC	This sets the OPC status bit when all pending operations have completed. Since your program can
	read this status bit on an interrupt basis, *OPC allows subsequent commands to be executed.

The trigger subsystem must be in the Idle state in order for the status OPC bit to be true. Therefore, as far as triggers are concerned, OPC is false whenever the trigger subsystem is in the Initiated state. However, OPC is also false if there are any commands still pending.

Note	For a detailed discussion of WAI, OPC and *OPC?, see "Device/Controller Synchronization
	Techniques" in ANSI/IEEE Std 488.2.

DFI (Discrete Fault Indicator)

Whenever a fault is detected in the power supply, it is capable of generating a FLT signal at the digital port (see "Appendix D - Digital Port Functions" in the power supply Operating Guide). The source for the DFI signal can be any Questionable, Operation, or Standard Event status event (see Figure 4-1).

RI (Remote Inhibit)

Whenever a remote inhibit signal is received at the digital port (see “Appendix D - Digital Port Functions” in the power supply Operating Guide), the power supply will receive an RI event at the Questionable Status register. By programming the status subsystem, you may use RI to generate a service request (SRQ) to the controller and/or to create a DFI output at the digital port. By using RI/DFI in this way, you can chain the power supplies to create a serial shutdown in response to the INH input.

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