Barracuda 18LP Family: ST39175LW/LC ST318275LW ... - Seagate

54 Barracuda 18LP Product Manual, Rev. E
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) and EPI standards as defined in American National Standard documents T10/
1302D and X3T10/1143D, and operate compatibly at the interface with devices that only support earlier singleended
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. Low voltage differential
(LVD) circuits are not compatible with high voltage differential (HVD).
The family of drives in this manual supports the single-ended and low voltage differential physical interconnects
(hereafter referred to as SE and LVD, respectively) as described in the ANSI SPI-2 and EPI standards.
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. Intermediate SCSI devices shall not be terminated. Models “LC” and “LW” have no 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 SPI-2 and EPI standards 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 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
Maximum transfer rate
Asynchronous Fast-5 Fast-10 Fast-20 Fast-40
SE “LC,” “LW” yes yes yes yes no
LVD “LC,” “LW” yes yes yes yes yes