Reselection Procedure; Reselection Timeout Procedure - Seagate SCSI Interface Product Manual

20
Reselection can be used only in systems that have Arbitration phase implemented.
The 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 assert-
ing 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 ini-
tiator'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 implemented,
the initiator shall not respond to Reselection if bad parity is detected. The initiator shall not respond to a Rese-
lection 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 meters 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.
3.1.4.2

Reselection timeout procedure

This Reselection timeout procedure is specified for clearing the SCSI bus during a Reselection phase. If the
target waits a minimum of a selection timeout period and there has been no BSY response from the initiator,
the target shall continue asserting SEL and I/O and shall release all Data Bus signals. If the target has not
detected BSY to be true after at least a selection abort time plus two deskew delays, the target shall release
SEL and I/O allowing the SCSI bus to go to the Bus Free phase. SCSI devices that respond to Reselection
shall ensure that the Reselection was still valid within a selection abort time of their assertion of BSY. Failure to
comply with this requirement could result in an improper Reselection (two initiators connected to the same tar-
get or the wrong initiator connected to a target).
If an initiator times out while waiting to be reselected, the initiator should attempt to select and issue Request
Sense to determine if the previous command is:
1. Still in process (Busy Status is returned),
2. Aborted with valid Request Sense data, or
3. Aborted without valid Request Sense data.
SCSI Interface Product Manual, Rev. J