Viper 60s Theory Of Operation And Maintenance Manual page 40

INTERFACE
4.7 Selection and Reselection Phases
The Selection and Reselection Phases provide a method for establishing a link between the
Initiator and Target. When selected by the Initiator (read or write data), 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, there are no restrictions on the sequences between information
transfer phases. A phase type may be followed by the same phase type. For example, a
data phase 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 Target's 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 commu-
nication.
As an example the Initiator can select a Viper drive to begin an operation, or the Viper
drive can re-select an Initiator to continue an operation previously disconnected.
For the Copy function, the Viper drive can act as an Initiator and select another controller
as a Target source or destination for the Copy operation.
The Selection and Reselection Phases can be terminated for either of the following
reasons:
• A Selection/Reselection time-out occurs. A Target or Initiator did not respond to a
Selection or Reselection Phase within a maximum abort time of 200 J15ec.
• A reset signal occurs on the SCSI bus, and all sequences are terminated and signals
released by all Targets and Initiators.
The Initiator can use the A TN signal to notify the Viper drive that an ID message from the
Initiator is ready. To ensure the Target recognizes the Attention condition before the
Command Phase is entered, the ATN signal must be low before the SEL is asserted and
BSY de-asserted.
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