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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 Ultra160
SCSI Parallel Interface-3 (SPI-3) standards as defined in American National Standard document X3T10/1302D
revision 14, and operate compatibly at the interface with devices that support earlier SCSI-2 and SCSI-3 stan-
dards. 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 SCSI 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 dynam-
ically 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. 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 X3T10/1302D, sections 6.4 and 6.5 for the maximum
number of devices that can successfully operate at various interface transfer rates on SE and LVD daisy
chains.
Table 12 shows the interface transfer rates supported by the various drive models defined in this manual.
Table 12:
Interface transfer rates supported
Interface type/
drive models
ST336918N
SE
LVD
ST336938LW
9.6.3
SCSI interface cable requirements
The characteristics of cables used to connect Ultra160 SCSI parallel interface devices are discussed in detail
in Section 6 of ANSI Standard X3T10/1302D. The cable characteristics that must be considered when inter-
connecting the drives described in this manual in a Ultra160 SCSI 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)
To minimize discontinuances and signal reflections, cables of different impedances should not be used in the
same bus. Implementations may require trade-offs in shielding effectiveness, cable length, number of loads
and spacing, transfer rates, and cost to achieve satisfactory system operation. If shielded and unshielded
cables are mixed within the same SCSI bus, the effect of impedance mismatch must be carefully considered.
Proper impedance matching is especially important in order to maintain adequate margin at FAST-20 and
FAST-40 SCSI transfer rates.
Note.
For LVD operation, twisted pair cables are recommended. For LVD Fast-40 operation, twisted pair
cables are strongly recommended. For Fast-80 operation, twisted pair cables are required .
Maximum transfer rate
Asynchronous Fast-5
yes
yes
yes
yes
Barracuda 36ES2 Product Manual, Rev. B
Fast-10
Fast-20
yes
yes
yes
yes
Fast-40
Fast-80
no
no
yes
yes
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