Seagate Ultra 320 - Manual (Page 110)

96 Parallel SCSI Interface Product Manual, Rev. A
If an abnormal condition prevents the target from responding with a PARALLEL PROTOCOL REQUEST mes-
sage or with a MESSAGE REJECT message, then both SCSI devices shall use the eight-bit/asynchronous
data transfer mode with all the protocol options bits set zero to indicate ST DATA IN and ST DATA OUT phases
between the two SCSI devices.
Following a target's responding PARALLEL PROTOCOL REQUEST message, an implied agreement for data
transfers shall not be considered to exist until:
a. the initiator receives the last byte of the PARALLEL PROTOCOL REQUEST message and parity is
valid; and
b. the target does not detect an attention condition on the last byte of the PARALLEL PROTOCOL
REQUEST message.
If the initiator does not support the target’s responding PARALLEL PROTOCOL REQUEST message’s values,
the initiator shall create an attention condition and the first message shall be a MESSAGE REJECT message.
If during the PARALLEL PROTOCOL REQUEST message the initiator creates an attention condition and the
first message of the MESSAGE OUT phase is either a MESSAGE PARITY ERROR or MESSAGE REJECT
message, the data transfers shall be considered to be negated by both SCSI devices. In this case, both SCSI
devices shall use the eight-bit asynchronous data transfer mode with all the protocol options bits set zero to
indicate ST DATA IN and ST DATA OUT phases for data transfers between the two SCSI devices.
4.3.13 QAS REQUEST
The QAS REQUEST message is sent from a target that has both information unit transfers and QAS enabled
to begin a QAS phase (see Section 3.1.2.3) after a DT DATA phase.
4.3.14 RESTORE POINTERS
The RESTORE POINTERS message is sent from a target to direct the initiator to copy the most recently saved
command, data, and status pointers for the task to the corresponding active pointers. The command and status
pointers shall be restored to the beginning of the present command and status areas. The data pointer shall be
restored to the value at the beginning of the data area in the absence of a SAVE DATA POINTERS message or
to the value at the point at which the last SAVE DATA POINTERS message occurred for that task.
When information unit transfers are enabled, there are implied restore pointers, and the target shall not trans-
mit a RESTORE POINTERS message. For more information on this see Section 6 and particularly Section
6.2.3.
4.3.15 SAVE DATA POINTERS
The SAVE DATA POINTERS message is sent from a target to direct the initiator to copy the current data
pointer to the saved data pointer for the current task.