Scsi Interface Physical Description - Seagate ST336732LC Product Manual

Cheetah X15 36LP Product Manual, Rev. D
9.6.2

SCSI interface physical description

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 SPI-4 standard. These drives imple-
ment driver and receiver circuits that can operate either SE or LVD. However, they cannot switch dynamically
between SE and LVD operation.
• ST336752 and ST318452 models support the physical interface requirements of the SPI-4 standard 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 Ultra160
protocol extensions that they don't support.
• ST336732 and ST318432 models support the physical interface requirements of the SPI-4 standard 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 typically operate on a daisy-chain interface in which other SCSI devices are also operating. Devices
on the daisy chain must all operate in the same mode, either SE or LVD, but not a mixture of these. On the
interface daisy chain, all signals are common between all devices on the chain, or bus, as it is also called. This
daisy chain of SCSI devices 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 termination circuits that can be enabled by installation of a jumper plug (TE) on the device. These ter-
mination circuits receive power from either a source internal to the device, or from a line in the interface cable
specifically powered 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 SPI-4 standard for the maximum number of devices
that can successfully operate at various interface transfer rates on SE and LVD daisy chains.
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 daisy chain 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 15 shows the interface transfer rates supported by the various drive models defined in this manual.
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