Viper 60s Theory Of Operation And Maintenance Manual page 55

VIPER SCSI 60, 125
&
150 THEORY OF OPERATION AND MAINTENANCE
4.22 Disconnect/Reconnect Function
When the drive is performing a task not requiring communication with the Initiator or
when the Viper drive determines there is long time with no bus activity, it disconnects
from the bus. For example, when repositioning the tape or when writing to tape from a full
buffer or when a SCSI disconnect boundary is reached (see Configuration Jumpers).
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 be capable of supporting messages
other than Command Complete. To disconnect from the bus, the Viper drive performs the
following procedure:
1. The drive can send a Save Data Pointers Message if the disconnecting function was
a data transfer.
2. The drive sends a Disconnect Message indicating it is going to disconnect.
3. The dri ve disconnects from the bus by deasserting 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 continuously polls the bus for selection by another
Initiator. If the Viper drive is selected while disconnected,
it
only allows the following
actions:
• The Viper drive accepts the command and immediately returns Busy Status. The
Viper drive does not queue commands.
• Immediately following the selection, the Initiator may send the Identify, No
Message, or Bus Device Reset messages to the drive.
When the disconnected drive is ready to reconnect with the Initiator,
it
does the following:
1. Monitors the bus waiting for a Bus Free Phase to occur. When a Bus Free Phase is
sensed, the Viper drive Arbitrates for the bus.
2. Wins arbitration. The Viper 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
Arbitra tion.
3. When the Viper drive has successfully reselected the Initiator, it sends an identify
message to reestablish the path between the drive and the Initiator. This message
is always SOh since the Viper drive is initiating the reselection and is always
LUN O. If the reconnecting function is a data transfer, a Restore Pointers Message
is implied when the Viper drive sends an identify message to the Initiator and the
Initiator responds accordingly.
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