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Cheetah 4LP Product Manual, Rev. B
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. See Table 12 for the maximum number of SCSI devices
(including the host) that may be daisy-chained together. The SCSI devices at both ends of the daisy-chain are
to be terminated. Do not terminate intermediate SCSI devices (see Figure 16). Remove the terminator enable
jumper TE on J2 select header ("N" and "W" models), or the external terminators ("WD" model), not the termi-
nator power source selector jumper TP (Figure 10).
"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 different are satisfactory. A characteristic impedance as listed in Table 12 is
recommended 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. Use a 50-conductor flat cable or 25 twisted pair cable. A mini-
mum 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. Use a 68-conductor flat cable or 34 twisted pair
cable 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.
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