Specifications

52 Hawk 2XL 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. A maximum of 8 (“N” models) or 16 (“W” and “WC”) SCSI
devices (including the Host) may be daisy chained together. 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 termina-
tor enable jumper TE on J2 select header (“N” and “W” models), or the external terminators (“WC” model), not
the terminator power source selector jumper TP (Figure 7d).
“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 cannot sup-
port 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.
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,
impedance’s that are somewhat lower are satisfactory. A characteristic impedance of 100 ohm ±10% is recom-
mended for unshielded flat cable or twisted pair ribbon cable. However, most available cables have a some-
what lower characteristic impedance. 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 effective-
ness, cable length, the number of loads, transfer rates, and cost to achieve satisfactory 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.
Model “N” uses 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
Model “W” uses 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 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.
9.6.3.1 Single-ended I/O circuits (“N”, “W” and “WC” models)
The maximum total cable length for use with drives having single-ended I/O driver and receiver circuits shall be
6 metres (19.7 ft.) when operating at line data transfer rates of 5 Mbytes/sec or less, and 3 metres (9.85 ft.)
when operating at transfer rates greater than 5 Mbytes/sec (FAST SCSI). A stub length of no more than 0.1
metre (0.33 ft.) is allowed off the mainline interconnection with any connected equipment. An ideal impedance
match with cable terminators implies a cable characteristic impedance of 132 ohms. Single-ended I/O cable
pin assignments are shown in Table 11a, 11b and 11c.
9.6.3.2 Differential I/O circuits (“ND”, “WD” and “DC” models)
The Hawk 2XL has no differential I/O models available.
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