Scsi Interface Physical Description; Scsi Interface Cable Requirements - Seagate ST373405LW - Cheetah 73.4 GB Hard Drive Product Manual

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9.6.2

SCSI interface physical description

The drive models described by this product manual support the physical interface requirements of the SCSI-3
Parallel Interface-3 (SPI-3) standards as defined in American National Standard document T10/1302D, 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 SCSI-3 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 ANSI SPI-3 standard. These drives
implement driver and receiver circuits that can operate either SE or LVD. However, they cannot switch dynami-
cally between SE and LVD operation.
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 Standard T10/1302D, sections 6.4 and 6.5 for the max-
imum 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 11 shows the interface transfer rates supported by the various drive models defined in this manual.
Table 11: Interface transfer rates supported
Interface type/
drive models
ST373405
SE LC
ST373405
LW
ST336605
LC
ST373405
LVD LC
ST373405
LW
ST336605
LC
9.6.3

SCSI interface cable requirements

The characteristics of cables used to connect SCSI-3 parallel interface devices are discussed in detail in sec-
tion 6 of ANSI Standard T10/1302D. The cable characteristics that must be considered when interconnecting
the drives described in this manual in a SCSI-3 parallel, daisy-chain interconnected system are:
• characteristic impedance (see Section 6.1)
• propagation delay (see Section 6.1)
• cumulative length (see Sections 6.4 and 6.5)
• stub length (see Sections 6.4 and 6.5)
• device spacing (see Sections 6.4 and 6.5)
Maximum transfer rate
Asynchronous Fast-5
yes yes
yes yes
Cheetah 73LP Product Manual, Rev. C
Fast-10 Fast-20
yes yes
yes yes
Fast-40 Fast-80
no no
yes yes