Disconnect Example - Seagate ST11200 Product Manual

50 Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
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4.4.2

Disconnect example

In the single command example, the length of time necessary to obtain the data may require a time consum-
ing physical seek. In order to improve system throughput, the disc drive may disconnect from the initiator,
freeing the SCSI bus to allow other requests to be sent to other SCSI devices. To do this, the initiator must be
reselectable and capable of restoring the pointers upon reconnection. The disc drive must be capable of
arbitrating for the SCSI bus and reselecting the initiator. See Figure 4.4-2.
After the disc drive has received the Read command (and has determined that there will be a delay), it
disconnects by sending a Disconnect message and releasing BSY (goes to BUS Free state).
When the data is ready to be transferred the disc drive reconnects to the initiator, the initiator restores the
pointers to their most recently saved values (which, in this case, are the initial values) and the disc drive
continues (as in the single command example) to finish the operation. The initiator recognizes that the opera-
tion is complete when a Command Complete message is received.
If the disc drive elects to disconnect after transferring part of the data (e.g. while crossing a cylinder bound-
ary), it sends a Save Data Pointer message and a Disconnect message to the initiator and then disconnects.
When reconnection is completed, the initiator restores the current data pointer to the value it was immedi-
ately before the Save Data Pointer message.