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Barracuda 9LP Product Manual, Rev. C
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-2 (SPI-2) standards as defined in American National Standard document X3T10/1142D, and
operate compatibly at the interface with devices that only 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 family of drives in this manual supports the single-ended, high voltage differential and low voltage differen-
tial physical interconnects (hereafter referred to as SE, HVD, and LVD, respectively) as described in the ANSI
SPI-2 standard. Drive models whose model numbers end with "N," "W," and "WC" implement SE drivers and
receivers. Drive models whose model numbers end in "WD" implement HVD drivers and receivers. Drive mod-
els whose model numbers end with "LC" and "LW" implement driver and receiver circuits that can operate
either SE or LVD (but never HVD).
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, LVD, or HVD, 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 oper-
ate correctly. Intermediate SCSI devices shall not be terminated. 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 termination circuits receive power from either a source internal to the device, or from a line in the inter-
face cable specifically powered for that purpose. On SE drive models "N" and "W," terminator power source can
be selected using the TP jumpers on the option select header J2. TE is also on the option select header J2 for
models "N" and "W." See Section 8.1 for details. Models "WC," "WD," "LC," and "LW" have no onboard termina-
tion 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 X3T10/1142D, sections 6.4
and 6.5 for the maximum number of devices that can successfully operate at various interface transfer rates on
SE, HVD, and LVD daisy chains.
"WC" model drives plug into PCB or bulkhead connectors in the Host. They may be connected in a daisy-chain
by the host backplane wiring or PCB circuit runs that have adequate DC current carrying capacity to support
the number of drives plugged into the PCB or bulkhead connectors. A single 80 pin I/O connector cable cannot
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 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
SE
"N," "W," "LC," "LW"
LVD
"LC," "LW"
HVD
"WD"
Maximum transfer rate
Asynchronous Fast-5
yes
yes
yes
yes
yes
yes
Fast-10
Fast-20
yes
yes
yes
yes
yes
yes
55
Fast-40
no
yes
no
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