Seagate Cheetah ST3146854, Руководство пользователя

Страница: 77 / 108

Cheetah 15K.4 SCSI Product Manual, Rev. D 67
9.6.2 SCSI interface physical description
Cheetah 15K.4 SCSI drives support the physical interface requirements of the Ultra320 SCSI Parallel Inter-
face-4 (SPI-4), and operate compatibly at the interface with devices that support earlier SCSI-2 and SCSI-3
standards. It should be noted that this is only true if the systems engineering has been correctly done, and if
earlier SCSI-2 and SCSI-3 devices respond in an acceptable manner (per applicable SCSI Standards) to reject
newer Ultra320 protocol extensions that they don’t support.
The drives documented in this manual support single-ended and low voltage differential physical interconnects
(hereafter referred to as SE and LVD, respectively) as described in the corresponding ANSI SPI document ref-
erenced in the preceeding paragraph. These drives implement driver and receiver circuits that can operate
either SE or LVD. However, they cannot switch dynamically between SE and LVD operation.
The drives typically operate on a bus with other SCSI devices. All devices on the bus must operate in the same
mode, either SE or LVD, but not a mixture of these. All signals on the bus are common between all devices on
the bus. This bus must be terminated at both ends with the proper impedance in order to operate correctly. Do
not terminate intermediate SCSI devices. In some cases, the SCSI devices at each end have onboard termina-
tion circuits that can be enabled by installation of a jumper plug (TE) on the device. These termination circuits
receive power from either a source internal to the device, or from a line in the interface cable specifically pow-
ered for that purpose. LC and LW model drives do not have onboard termination circuits. Some type of external
termination circuits must be provided for these drives by the end user or designers of the equipment into which
the drives will be integrated. See the ANSI T10 Standard referenced above for the maximum number of
devices that can successfully operate at various interface transfer rates on SE and LVD buses.
LC model drives plug into PCBA or bulkhead connectors in the host. They may be connected in a daisy chain
by the host backplane wiring or PCBA circuit runs that have adequate DC current carrying capacity to support
the number of drives plugged into the PCBA or bulkhead connectors. A single 80-pin I/O connector cable can-
not support the DC current needs of several drives, so no cables beyond the bulkhead connectors should be
used. A single drive connected via a cable to a host 80-pin I/O connector is not recommended.
Table 2 shows the interface transfer rates supported by the various drive models defined in this manual.
9.6.3 SCSI interface cable requirements
The characteristics of cables used to connect parallel interface drives are discussed in the ANSI standards ref-
erenced in this section.
9.6.3.1 Cable requirements
The characteristics of cables used to connect Ultra320 SCSI parallel interface devices are discussed in detail
in Section 6 of ANSI Standard T10/1365D Rev. 3. The cable characteristics that must be considered when
interconnecting the drives described in this manual in a Ultra320 SCSI parallel, daisy-chain interconnected
system are:
• characteristic impedance (see T10/1365D Sections 6.3.3 and 6.3.4)
Table 2:
Interface transfer rates supported
Interface type/
drive models
Maximum transfer rate
Asynchronous Fast-5 Fast-10
Fast-20
(Ultra)
Fast-40
(Ultra2)
Fast-80
(Ultra160)
Fast-160
(Ultra320)
SE ST3146854LW/LC
ST373454LW/LC
ST336754LW/LC
yes yes yes yes no no no
LVD ST3146854LW/LC
ST373454LW/LC
ST336754LW/LC
yes yes yes yes yes yes yes