Multiple Connections; Background - Tandberg Data SLR7-140 PROGRAMMER Manual

Tandberg Data Commands
4.6.

Multiple Connections

4.6.1.

Background

While the SCSI-bus is in the BUS FREE state, any Initiator
may attempt a connection to the Drive. In most cases this
will happen when the Drive has completed execution of some
previous command (a STATUS byte and a COMMAND COMPLETED
message has been sent). As soon as the SCSI-bus becomes
free, an Initiator may again select the Drive. The Drive
will respond to the selection and a new command may be
transferred and later executed by the Drive. This is the
usual scenario where commands are executed in a serial
manner. The connecting Initiators may be the same or
different Initiators.
If allowed, the Drive may disconnect from the current
Initiator during command execution. The Drive reconnects
automatically at a later stage, but this will temporarily
leave the SCSI-bus in the BUS FREE state, even if the
current command has not completed its execution. During
these periods of BUS FREE phases, any Initiator (including
the original), may seize the opportunity to select the
Drive. In a multiple-initiator system, one Initiator may
not even know that another Initiator already has
established a connection to the Drive. When the Drive is
logically connected to two Initiators at the same time (or
the same Initiator twice), a concurrent command situation
exists.
Regardless of whether a concurrent command situation exists
or not, the different (or same) Initiator may connect to
the same or different LUNs within the Drive. Even if the
Drive only has a single LUN (LUN 0 (zero)), attempts may be
made to connect to other LUNs.
4-9 Tandberg SLR Product Line SCSI Functional Specifications