
Page 10 DDS-4 SCSI Interface Manual, Rev. B
For example, the Initiator can select a tape drive to begin an operation, or the tape
drive can reselect an Initiator to continue an operation previously disconnected. The
tape drive does not disconnect from the bus unless the host asserted the host ID bit
during selection and the host sent an Identify message of C0h.
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 Sec.
• A reset signal occurs on the SCSI bus. All sequences are terminated, and
signals are released by all Targets and Initiators.
The Initiator can use the ATN signal to notify the DAT drive that an IDENTIFY
message from the Initiator is ready. To ensure that the Target recognizes the
Attention condition before the Command Phase is entered, the ATN signal must be
low before SEL is asserted and BSY de-asserted.
Select With Attention
The host system can select the drive only using Select With Attention. The Select
With Attention allows the host to send the Identify message to specify that the host
supports disconnect.
The host can also use the Select With Attention to send messages other than the
Identify message. For example, a Bus Device Reset or Abort message can be sent
to cause the drive to reset itself without affecting any other device on the SCSI bus.
The DAT drive responds appropriately to the host messages.
If the drive receives an illegal message, it enters the Message Out Phase and sends
the Reject message to the host. If any reserved bit (bits 5, 4, or 3) is set, an Identify
message is rejected.
Responses to Identify messages with a non-zero LUN are described in the following
subsection.
Identify Message
Because the drive only supports Logical Unit 0, the Identify message is used solely
to specify the Disconnect option. The bit map for the Identify message is as follows:
Bits 7 6 5 4 3 2 1 0
| DIS | Reserved | LUN
If bit 7 is set, the Identify message is indicated. Bit 6 (shown as DIS) is set as
follows:
0 The host does not support disconnects. The drive does not disconnect from the
SCSI bus during the current command.
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