
Tandberg Data
Hardware Interface
___________________________________________________________________________
8-10
Tandberg SLR Reference Manual
8.4.5. Reselection Phase
Disconnection is used by the Target to let other devices use the SCSI-bus during time
consuming operations. The Reselect option is activated if the Initiator issues an Identify
message - with the Disconnect Privilege bit set to one - immediately following the Selection
Phase. (See Section 8.4.4). Hereby the Initiator signals that the Drive is allowed to discon-
nect. If the Target decides that a particular command will consume a lot of time, it will send
the Disconnect Message In, and release the bus. When the Target wants contact again, it will
have to go through the Reselection Phase. This Phase is similar to the Selection Phase,
except that the I/O line is asserted.
Before the Target can reselect, it must first go through the Arbitration
Phase to gain control of the bus. Then the following sequence takes
place:
Having gained control
of the bus, the Target
can enter the Reselec-
tion Phase by assert-
ing SEL and I/O.
p
The Initiator responds
to the reselection by
asserting BSY.
{
Then the Initiator waits
for the Target to deas-
sert SEL before it
again deasserts BSY.
-BSY
-SEL
-ATN
-MSG
-C/D
-I/O
-REQ
-ACK
Bus
Free
Arbitration
Re-
selection
Message In
Arbit. ID
Mess. In Byte
Selection IDs
min. 1.2
µs
max. 200
ms
-DB(0-7, P)
DB(8-15, P1)
Figure 8.4 Arbitration, Reselection and Message In Phases
Since the Target will assert BSY before it deasserts SEL, the “or-tied” BSY line remains
asserted, even if the Initiator deasserts BSY.
The Reselection Phase will always be followed by a Message In Phase that will report Identify
Message. This helps the Initiator to see which device that reselects if the Initiator should have
issued commands to more than one device.