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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-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 and low voltage differential physical interconnects
(hereafter referred to as SE 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 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 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. 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," "LC," and "LW" have no onboard termination cir-
cuits. 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 and
LVD daisy chains.
"LC" and "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 recom-
mended.
Table 14 shows the interface transfer rates supported by the various drive models defined in this manual.
Table 14:
Interface transfer rates supported
Interface type/
drive models
SE
"N," "W," "LC," "LW"
LVD
"LC," "LW"
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 X3T10/1142D. The cable characteristics that must be considered when interconnect-
ing 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
Medalist Pro Product Manual, Rev. D
Fast-10
Fast-20
yes
yes
yes
yes
Fast-40
no
yes
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