Reselection Phase - Seagate ST11200 Product Manual

Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
_____________________________________________________________________________________
3.1.4

Reselection phase

Reselection is a phase that allows a target to reconnect to an initiator for the purpose of continuing some
operation that was previously started by the initiator but was suspended by the target (i.e., the target discon-
nected by allowing a Bus Free phase to occur before the operation was complete).
Reselection can be used only in systems that have Arbitration phase implemented.
The disc drive implements the Reselection phase if the system is capable of supporting Reselection.
ATN during Selection implies that the host supports messages other than command complete.
Bit 6 of the Identify message indicates that the Disconnected/Reconnect privilege is granted.
ATN not asserted during Selection means that the host doesn't support the Identify message, so the Discon-
nected/Reconnect privilege cannot be granted. If a target is not granted disconnect/reconnect privileges, it
shall not disconnect until the current command is completed. None of the Disconnect/Reconnect Control
Page (page 02h) parameters of Mode Select are applicable if disconnect/reconnect privileges are not granted
by the initiator. Operations taking longer than the bus inactivity limit specified do not cause a disconnect.
3.1.4.1 Reselection Procedure
Upon completing the Arbitration phase, the winning SCSI device has both BSY and SEL asserted and has
delayed at least a bus clear delay plus a bus settle delay. The winning SCSI device becomes a target by
asserting the I/O signal. That device shall also set the Data Bus to a value that is the OR of its SCSI ID bit and
the initiator's SCSI ID bit. The target shall wait at least two deskew delays and release BSY. The target shall
then wait at least a bus settle delay before looking for a response from the initiator.
The initiator shall determine that it is reselected when SEL, I/O, and its SCSI ID bit are true and BSY is false
for at least a bus settle delay. The reselected initiator may examine the Data Bus to determine the SCSI ID of
the reselecting target.
The reselected initiator shall then assert BSY within a selection abort time of its most recent detection of
being reselected; this is required for correct operation of the timeout procedure. In systems with parity imple-
mented, the initiator shall not respond to Reselection if bad parity is detected. The initiator shall not respond
to a Reselection if more than two SCSI ID bits are on the Data Bus.
After the target detects BSY, it shall also assert BSY and wait at least two deskew delays and then release
SEL. The target may then change the I/O signal and the Data Bus. After the reselected initiator detects SEL
false, it shall release BSY. The target shall continue asserting BSY until the target is ready to relinquish the
SCSI bus.
Note. When the target is asserting BSY, a transmission line phenomenon known as a "Wired-OR
glitch" may cause BSY to appear false for up to a round trip propagation delay following the
release of BSY by the initiator. This is the reason why the Bus Free phase is recognized only
after both BSY and SEL are continuously false for a minimum of a bus settle delay. Cables
longer than 25 metres should not be used even if the chosen driver, receiver, and cable
provide adequate noise margins, because they increase the duration of the glitch and could
cause SCSI devices to inadvertently detect the Bus Free phase.
15