Seagate Ultra 320 - Manual (Page 28)

14 Parallel SCSI Interface Product Manual, Rev. A )
The Host Adapter/Initiator must be identified by one of the 16 SCSI Device IDs (usually 7, which is highest arbi-
tration priority). Make sure that none of the devices on the SCSI bus have duplicate IDs.
Certain SCSI bus functions are assigned to the initiator and certain SCSI bus functions are assigned to the tar-
get. The initiator will select a particular target. The target will request the transfer of Command, Data, Status, or
other information on the data bus.
Under SCSI-2 protocol, information transfers on the data bus are interlocked and follow a defined REQ/ACK
Handshake protocol. One byte of information will be transferred with each handshake. Synchronous data
transfers do not require a one-for-one interlocking of REQ/ACK signals, but the total number of REQ pulses in
a particular data transfer event must equal the total number of ACK pulses. The synchronous data transfer
option is described in Paragraphs 3.1.5.2 and 3.5.3.2 of the Ultra2 SCSI Interface Manual, part number
77738479.
This Ultra160/Ultra320 Parallel SCSI Interface Manual, discusses SCSI-3 protocol (now called only “SCSI”).
There are now different ways of latching data from the REQ and ACK signals, depending on whether ST DATA
phases, DT DATA phases, or paced transfers are being used for information transfers. This is described in
Section 2.1.2.
The drive supports single initiator, single target; single initiator, multiple target; multiple initiator, single target; or
multiple initiator, multiple target bus configurations.
Table 1: SCSI IDs and associated SCSI bus arbitration priorities
SCSI
address
DB
15
DB
8
DB
7
DB
0
Priority
7 - - - - - - - - 1 - - - - - - - 1
6 - - - - - - - - - 1 - - - - - - 2
5 - - - - - - - - - - 1 - - - - - 3
4 - - - - - - - - - - - 1 - - - - 4
3 - - - - - - - - - - - - 1 - - - 5
2 - - - - - - - - - - - - - 1 - - 6
1 - - - - - - - - - - - - - - 1 - 7
0 - - - - - - - - - - - - - - - 1 8
15 1 - - - - - - - - - - - - - - - 9
14 - 1 - - - - - - - - - - - - - - 10
13 - - 1 - - - - - - - - - - - - - 11
12 - - - 1 - - - - - - - - - - - - 12
11 - - - - 1 - - - - - - - - - - - 13
10 - - - - - 1 - - - - - - - - - - 14
9 - - - - - - 1 - - - - - - - - - 15
8 - - - - - - - 1 - - - - - - - - 16
Key: - = a logical 0 bit resulting from the data bus bit being released