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78 Chapter 2
Programming Fundamentals
Using the Instrument Status Registers
What are the Status Register SCPI Commands? Most monitoring
of the instrument conditions is done at the highest level using the IEEE
common commands indicated below. Complete command descriptions
are available in the IEEE commands section at the beginning of the
language reference. Individual status registers can be set and queried
using the commands in the STATus subsystem of the language
reference.
*CLS (clear status) clears the status byte by emptying the error
queue and clearing all the event registers.
*ESE, *ESE? (event status enable) sets and queries the bits in the
enable register part of the standard event status register.
*ESR? (event status register) queries and clears the event register
part of the standard event status register.
*OPC, *OPC? (operation complete) sets the standard event status
register to monitor the completion of all commands. The query stops
any new commands from being processed until the current
processing is complete, then returns a ‘1’.
*SRE, *SRE? (service request enable) sets and queries the value of
the service request enable register.
*STB? (status byte) queries the value of the status byte register
without erasing its contents.
Why Would You Use the Status Registers?
Your program often needs to be able to detect and manage error
conditions or changes in instrument status. There are two methods you
can use to programmatically access the information in status registers:
• The polling method
• The service request (SRQ) method
In the polling method, the instrument has a passive role. It only tells
the controller that conditions have changed when the controller asks
the right question. In the SRQ method, the instrument takes a more
active role. It tells the controller when there has been a condition
change without the controller asking. Either method allows you to
monitor one or more conditions.
The polling method works well if you do not need to know about
changes the moment they occur. The SRQ method should be used if you
must know immediately when a condition changes. To detect a change
using the polling method, the program must repeatedly read the
registers.