Product manual

Barracuda 4LP Product Manual, Rev. D 55
9.6.2 SCSI Interface physical description
The drives may be daisy chained together or with other compatible SCSI devices. Both ends of the cable must
be terminated. The “N,” “W,” and “WC” model drives implement single-ended drivers and receivers. All signals
are common between all SCSI devices. The drive may be daisy chained only with SCSI devices having the
same type drivers and receivers. Devices having single-ended interface circuits cannot be on the same daisy-
chain with devices having differential interface circuit. A maximum of 8 (“N” models) or 16 (“W” and “WC”) SCSI
devices (including the Host) may be daisy chained together. However, please note the restrictions described in
Section 9.6.3.1 about the number of devices allowed in a daisy chain. The SCSI devices at both ends of the
daisy-chain are to be terminated. Intermediate SCSI devices shall not be terminated (see Figure 10). Remove
the terminator enable jumper TE on J2 select header (“N” and “W” models), or the external terminators (“WD”
model), not the terminator power source selector jumper TP (Figure 7d).
“WC” and “DC” 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.
9.6.3 SCSI Interface Cable requirements
In general, cables having the characteristic impedances given in Section 9.6.3.1 are not available; however,
impedances that are somewhat lower are satisfactory. A characteristic impedance as listed in Table 12 is rec-
ommended for unshielded flat cable or twisted pair ribbon cable. 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, the number of loads, transfer rates, and cost to achieve sat-
isfactory 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 SCSI transfer rates.
Models “N” use nonshielded cable connectors. A 50 conductor flat cable or 25 twisted pair cable shall be used.
A minimum conductor size of 28 AWG should be used to minimize noise effects.
Suggested nonshielded flat cable part numbers are:
Flat cable - 35M-3365-50 Twisted pair - Spectra Twist in flat 455-248-50
Models “W” and “WD” use nonshielded cable connectors. A 68 conductor flat cable or 34 twisted pair cable
shall be used with connectors listed in 9.6.4. A minimum conductor size of 28 AWG should be used to minimize
noise effects.
Suggested nonshielded flat cable part numbers are:
Flat cable - 35M-3365-68 Twisted pair - Spectra Twist in flat 455-248-68
For “WC” and “DC” models:
The 80 pin connector option is intended for use on drives that plug directly into a PCB or wall/bracket mounted
connector in the host equipment. Installations with connectors on cables are not recommended.