Agilent Technologies 54621A Programmer's Manual page 68

Status Reporting
Figure 6-1 is an overview of the oscilloscope's status reporting structure.
The status reporting structure allows monitoring specified events in the
oscilloscope. The ability to monitor and report these events allows
determination of such things as the status of an operation, the availability
and reliability of the measured data, and more.
• To monitor an event, first clear the event, then enable the event. All
of the events are cleared when you initialize the instrument.
• To generate a service request (SRQ) interrupt to an external
controller, enable at least one bit in the Status Byte Register.
The Status Byte Register, the Standard Event Status Register group, and
the Output Queue are defined as the Standard Status Data Structure
Model in IEEE 488.2-1987.
IEEE 488.2 defines data structures, commands, and common bit
definitions for status reporting. There are also instrument-defined
structures and bits.
The bits in the status byte act as summary bits for the data structures
residing behind them. In the case of queues, the summary bit is set if the
queue is not empty. For registers, the summary bit is set if any enabled
bit in the event register is set. The events are enabled with the
corresponding event enable register. Events captured by an event
register remain set until the register is read or cleared. Registers are read
with their associated commands. The *CLS command clears all event
registers and all queues except the output queue. If you send *CLS is
sent immediately after a program message terminator, the output queue
is also cleared.
6-2