User's Guide

Page 26 DDS-4 SCSI Interface Manual, Rev. B
Disconnect/Reconnect Function
When the drive is performing a task not requiring communication with the Initiator or
when the DAT drive determines that a relatively long time has passed with no bus
activity, it disconnects from the SCSI bus. Examples are:
• When rewinding the tape.
• When writing to the tape and the buffer is full.
• When reading from the tape and the buffer is empty.
• When spacing, locating, or generally performing any tape motion when data
cannot be transferred on the SCSI bus.
During the time the Target is disconnected for one of these functions, the bus is free
for use by other devices. Both disconnect and reconnect are initiated by the Target.
When the Initiator first selects the drive, it sends an Identify Message indicating that
it is allowing the drive to disconnect and reconnect and to be capable of supporting
messages other than Command Complete (and the host set its own ID during
selection). To disconnect from the bus, the DAT drive performs the following
procedure:
1. The drive can send a Save Data Pointers Message if the disconnection function
was a data transfer.
2. The drive sends a Disconnect Message indicating it is going to disconnect.
3. The drive disconnects from the bus by de-asserting BSY and releasing control
of all bus signals.
The bus is now free for an Initiator to select any device on the bus, including the
drive that initiated the disconnect. The drive will respond to selection by another
Initiator.
If the DAT drive is selected while disconnected, it only allows the following actions:
• If the command is from a different initiator or is from the same initiator but to a
different LUN, the DAT drive accepts the command and immediately returns
Busy Status.
• Immediately following the selection, the Initiator may send the Identify, No Op,
Abort, or Bus Device Reset messages to the drive.
• If the command is from the same initiator to the same LUN, the current
command terminates with a Check Condition and an Abort Sense Key.
When the disconnected drive is ready to reconnect with the Initiator, it does the
following.
1. It monitors the bus waiting for a Bus Free Phase to occur. When a Bus Free
Phase is sensed, the DAT drive arbitrates for the bus.
2. If it wins arbitration, the DAT drive then attempts to reselect the Initiator. If the
Initiator fails to respond in 250 mSec, the drive drops all bus signals and allows
the bus to again enter the Bus Free Phase. The drive then repeats the
attempted Arbitration.