Scsi Bus; Figure 3. Scsi Id Bits - Seagate SCSI Interface Product Manual

SCSI Interface Product Manual, Rev. J
2.0

SCSI bus

This manual discusses only the "logical" and timing characteristics of the SCSI system and interface. The SCSI
bus physical characteristics (voltages, connector configurations, pinouts, etc.) are given in the individual drive's
Product Manual, Volume 1, in the "Interface requirements" section, which covers all of the interface require-
ments and SCSI features supported by the drive described in the particular Product Manual being referenced.
Communication on the SCSI Bus is allowed between only two SCSI devices at a time. Some Seagate drives
support systems with a maximum of eight SCSI devices including the host computer(s) connected to the SCSI
bus. Some Seagate drives support systems with a maximum of sixteen SCSI devices on the SCSI bus. Each
SCSI device has a SCSI ID Bit assigned as shown in Figure 3. The SCSI ID is assigned by installing from 0 to
3 (8 device systems) jumper plugs or 0-4 (16 device systems) jumper plugs onto a connector in a binary coded
configuration during system configuration. Some drive models have an interface that includes the SCSI bus ID
lines, so that the host can set the drive ID over the interface (see individual drive's Product Manual, Volume 1,
"Option/configuration headers" section). Drives that support SCAM protocol (SCSI Configured Automatically)
can have their drive ID assigned via interface also (see Appendix A).
When two SCSI devices communicate on the SCSI Bus, one acts as an initiator and the other acts as a target.
The initiator (typically a host computer) originates an operation and the target performs the operation. The
drive always operates as a target, unless specified otherwise (i.e., certain commands are supported) in the
individual drive's Product Manual, Volume 1.
DB(7) DB(6)
SCSI ID = 6
SCSI ID = 7
Additional SCSI ID bits for devices that support 16 devices on the SCSI bus.
DB(15) DB(14)
SCSI ID = 14
SCSI ID = 15
Figure 3. SCSI ID bits
The Host Adapter/Initiator must be identified by one of the eight SCSI Device Addresses. Make sure that none
of the devices on the SCSI bus have duplicate addresses.
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.
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. Synchronous data transfer option is described in Paragraph
3.1.5.2 and 3.5.3.2.
The drive supports single initiator, single target; single initiator, multiple target; multiple initiator, single target; or
multiple initiator, multiple target bus configurations.
DB(5) DB(4)
SCSI ID = 4
SCSI ID = 5
DB(13) DB(12)
SCSI ID = 12
SCSI ID = 13
DB(3) DB(2)
SCSI ID = 2
SCSI ID = 3
DB(11) DB(10)
SCSI ID = 10
SCSI ID = 11
DB(1) DB(0)
SCSI ID = 0
SCSI ID = 1
DB(9) DB(8)
SCSI ID = 8
SCSI ID = 9
11
DATA BUS
DATA BUS