From Bob Pentecost  12/22/88                            X3T9.2/88-168

                 Asynchronous Event Notification (AEN)
                       Initialization Procedure
.
A target device that plans to use AEN capabilities, must first execute
an AEN initialization procedure.  This procedure allows the target
device to determine which devices on the SCSI bus are capable and
willing to receive an AEN.  Control of AEN within the target device is
determined by the Mode page 0Ah "Common Device-Type Control
Parameters"; therefore, if AEN is not permitted, this initialization
routine does not need to be performed.
.
This procedure is executed at power on time (after waiting the
recommended 10 seconds for all devices to be able to respond and
waiting the time specified by the "Common Device-Type Control
Parameters"), after a hard reset, when the target becomes aware of a
previous non-responding device, when the a Mode Select is executed
that changes the AEN control bits, or when the device wishes to report
an asynchronous event.
.
The target device that wishes to perform an AEN must determine which
devices on the bus are capable of receiving an AEN.  This is
accomplished by the target device becoming a temporary initiator and
polling each device that responds to selection. To poll a device, the
target (as a temporary initiator) attempts to select a device and if
the device responds to selection, the Verify State Test (see appendix
H.2.4.) is performed.  If the Verify State Test fails, then an AEN is
not sent to that device.  If the Verify State Test passes, then an
INQUIRY Command is performed on logical unit 0.  The Inquiry Data is
checked and if the Peripheral Qualifier is 0 (peripheral is connected)
and the Peripheral Device Type is 03h (Processor Device) then the AENC
bit is checked.  If the AENC bit is 0, then an AEN is not sent to that
device.  If the AENC bit is 1, then an AEN may be sent to the device.
.
If an initiator wishes to inhibit all AEN initialization polling,
there are several methods that may be used.  The target's default mode
parameters can disable AEN permission (this might be done by a
hardware switch that controls the default mode) or the initiator can
set the target's saved mode pages to disable AEN permission.