Selection Phase; Nonarbitrating System; Arbitrating Systems; All Systems - Seagate SCSI Interface Product Manual

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The procedure for a SCSI device to obtain control of the SCSI bus is as follows:
1. The SCSI device shall first wait for the Bus Free phase to occur. The Bus Free phase is detected when
BSY and SEL are simultaneously and continuously negated for a minimum of a bus settle delay. (Imple-
mentors note: This bus settle delay is necessary because a transmission line phenomenon known as a
"Wire-OR glitch" may cause BSY to briefly appear negated, even though it is being asserted.)
2. The SCSI device shall wait a minimum of a bus free delay after detection of the Bus Free phase (i.e., after
BSY and SEL are both negated for a bus settle delay) before driving any signal.
3. Following the bus free delay in Step 2, the SCSI device may arbitrate for the SCSI bus by asserting both
BSY and its own SCSI ID, however the SCSI device shall not arbitrate (i.e., assert BSY and its SCSI ID) if
more than a bus settle delay has passed since the Bus Free phase was last observed. (Implementors
Note: There is no maximum delay before asserting BSY and the SCSI ID following the bus free delay in
Step 2 as long as the bus remains in the Bus Free phase. However, SCSI devices that delay longer than a
bus settle delay plus a bus set delay from the time when BSY and SEL are first negated may fail to partici-
pate in arbitration when competing with faster SCSI devices.)
4. After waiting at least an arbitration delay (measured from its assertion of BSY) the SCSI device shall exam-
ine the Data Bus. If a higher priority SCSI ID bit is true on the Data Bus [DB(7) is the highest], the SCSI
device has lost the arbitration and the SCSI device must release its signals and return to Step 1. If no
higher priority SCSI ID bit is true on the Data Bus, the SCSI device has won the arbitration and it shall
assert SEL. Any other SCSI device that is participating in the Arbitration phase has lost the arbitration and
shall release BSY and its SCSI ID bit within a bus clear delay after SEL becomes true. A SCSI device that
loses arbitration may return to Step 1.
5. The SCSI device that wins arbitration shall wait at least a bus clear delay plus a bus settle delay after
asserting SEL before changing any signals.
See also paragraph 3.1.3.6 for arbitration in a SCAM compliant system.
Note. The SCSI ID bit is a single bit on the Data Bus that corresponds to the SCSI device's unique SCSI
address. All other seven Data Bus bits shall be released by the SCSI device. Parity is not valid during
the Arbitration phase, DB(P) may be undriven or driven to the true state, but shall not be driven to the
false state.
3.1.3

Selection phase

The Selection phase allows an initiator to select a target for the purpose of initiating some target function (e.g.,
Read or Write command).
Note. During the Selection phase the I/O signal shall be negated so this phase can be distinguished from the
Reselection phase.
3.1.3.1

Nonarbitrating system

In systems with the Arbitration phase not implemented, the initiator shall first detect the Bus Free phase and
then wait a minimum of a bus clear delay. Then, except in certain single initiator environments with initiators
employing the single initiator option (see Section 3.1.3.4), the initiator shall assert the desired target's SCSI ID
and its own initiator SCSI ID on the Data Bus. After two deskew delays, the initiator shall assert SEL.
3.1.3.2

Arbitrating systems

In systems with the Arbitration phase implemented, the SCSI device that won the arbitration has both BSY and
SEL asserted and has delayed at least a bus clear delay plus a bus settle delay before ending the Arbitration
phase. The SCSI device that won the arbitration becomes an initiator by releasing I/O. Except in certain single
initiator environments with initiators employing the single initiator option (see Section 3.1.3.4), the initiator shall
set the Data Bus to a value which is the OR of its SCSI ID bit and the target's SCSI ID bit. The initiator shall
then wait at least two deskew delays and release BSY. The initiator shall then wait at least a bus settle delay
before looking for a response from the target.
3.1.3.3

All systems

In all systems, the target shall determine that it is selected when SEL and its SCSI ID bit are true and the BSY
and I/O signals are false for at least a bus settle delay. The selected target will examine the Data Bus in order
SCSI Interface Product Manual, Rev. J