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SCSI Interface Description Page 9

The SCSI device waits a minimum of one bus free delay of 800 nSec

after detection of a Bus Free Phase before driving any signal. The Bus

Free Phase occurs after BSY and SEL are both false for a bus settle

delay of 400 nSec.

Following the 800 nSec Bus Free Delay in Step 2, the SCSI device

Arbitrates for the SCSI bus by asserting both BSY and its own SCSI ID;

however, the SCSI device does not assert a BSY and its SCSI ID if

more than a bus settle delay (1.8 Sec) has passed since the Bus Free

Phase was last observed.

After waiting at least an Arbitration delay (2.2 Sec) measured after

asserting a BSY signal, the SCSI device examines the data bus. If a

higher priority SCSI ID bit is true, then the SCSI device loses the

Arbitration. The SCSI device releases its signals and returns to Step 1. If

no higher priority SCSI ID bit is true on the bus, then the SCSI drive

wins the Arbitration and asserts the SEL signal. Any other device that

participated in the Arbitration Phase and lost Arbitration releases BSY

and its SCSI ID bit within a bus clear delay after SEL becomes true. A

SCSI device that loses Arbitration returns to Step 1.

The SCSI device that wins Arbitration waits at least one bus clear delay

plus a bus settle delay of 1,200 nSec after asserting the SEL signal and

changing the condition of other signals.

NOTE: The single SCSI ID bit on the data bus corresponds to the unique ID code

of the SCSI device. All other SCSI data bus bits are released by the SCSI device.

Parity is not valid during the Arbitration Phase. During the Arbitration Phase, DB(P)

may be undriven or driven true, but not false.

Selection and Reselection Phases

The Selection and Reselection Phases provide a method for establishing a link

between the Initiator and Target. When selected by an Initiator that supports

disconnects, the Target has the option of disconnecting from the SCSI bus. When

the Target needs to again establish the link to its original Initiator, the Target

reselects that Initiator.

When selection is made, no restrictions on the sequences between Information

Transfer Phases exist. A phase type may be followed by the same phase type. For

example, a data phase may be followed by another data phase.

A device that wins Arbitration assumes the roll of Initiator by releasing the I/O signal.

The Initiator sets the data bus to a value that is the OR of its SCSI ID bit and the

Targets ID bit. The Initiator waits at least two deskew delays (90 nSec) before

releasing the BSY signal and an additional bus settle delay (400 nSec) before

looking for a response from the Target.

A device that loses Arbitration must release the Select ID and BSY.

Operation enters the Selection or Reselection Phases when a particular device wins

a request in Arbitration and gains control of the SCSI bus. These phases allow the

device in control of the bus to select another specific device connected to the SCSI

bus for communication.

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