Unit Attention Condition - Seagate ST11200 Product Manual

Product Manual - Disc Drive SCSI-2 Interface (Vol. 2; Ver. 2), Rev. E
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Unit attention condition

The disc drive sets up the Unit Attention condition when it stores (within itself) a Unit Attention condition flag
for each device on the SCSI bus having an initiator relationship with the disc drive, and this Unit Attention
condition persists for each initiator until the condition is cleared (flag negated) by each initiator individually.
The Unit Attention condition results when one of the following events occur:
1. A power-on sequence occurs.
2. A reset is generated internally by the disc drive (caused by a power glitch).
3. A Bus Device Reset message causes the disc drive to reset itself.
4. The RESET I/O line resets the disc drive.
5. An initiator changes one or more of the Mode Select parameters in the disc drive (these changes could
affect one or more of the other initiators).
6. The inquiry data has been changed.
7. The mode parameters in effect for an initiator have been restored from non-volatile memory.
8. An event occurs that requires the attention of the initiator.
9. A Clear Queue message received.
10. The Log parameters are changed. Unit Attention Condition is posted for all initiators in the system other
than the one that changed the Log Parameters.
The Unit Attention Parameters page (page 00h, bit 4 of byte 2) of the Mode Select Command controls
whether or not a Check Condition Status is to be reported to affected initiators when a Unit Attention condition
exists. See Table 5.2.1-31.
The Unit Attention condition for a particular initiator is cleared when that initiator does one of the following:
1. It sends a Request Sense Command.
2. It sends any other legitimate command, with the exception of the Inquiry command. The Inquiry com-
mand does not clear the Unit Attention condition.
When a Unit Attention condition flag is stored in the disc drive for an initiator, the commands that initiator
issues to the disc drive operate as described in the following paragraphs.
If an initiator sends an Inquiry command to the disc drive when the disc drive has stored a Unit Attention
condition flag for that initiator before or after the disc drive reports Check Condition status), the disc drive
shall perform the Inquiry command and shall not clear the Unit Attention condition.
If an initiator sends a Request Sense command to the disc drive when a Unit Attention condition flag is stored
for that initiator (before or after the disc drive reports Check Condition), the disc drive shall discard any
pending sense data, report the Unit Attention Sense Key, and clear the Unit Attention condition (negate the
flag) for that initiator.
If an initiator issues a command other than Inquiry or Request Sense while a Unit Attention condition flag is
stored for that initiator, the disc drive may or may not perform the command and report Check Condition
status, depending on whether or not the Unit Attention bit is zero or one in the Unit Attention Mode Param-
eters page (Page 00h, bit 4 of byte 2). See Table 5.2.1-31. If a Request Sense is issued next, the Unit
Attention condition is reported and cleared (flag negated) as noted in the preceding paragraph. If another
command other than Request Sense or Inquiry is issued instead, the disc drive shall perform the command
and return the appropriate status. The Unit Attention condition for the subject initiator is cleared (flag negated)
and the sense data and flag indicating there has been a Unit Attention condition are lost.
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