Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
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3.1.2
Arbitration phase
The Arbitration phase allows one SCSI device to gain control of the SCSI bus so that it can assume the role
of an initiator or target. The disc drive arbitrates for the bus as a target implementing reselection or when
performing AEN (if AEN is implemented). The disc drive supports arbitration by multiple SCSI devices.
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.
(Implementors 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 participate 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
examine 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 arbitra-
tion 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.
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.
