Seagate HAWK 4 Series Product Manual page 57

Product Manual - Hawk 4 Family (Wide) SCSI-2 (Volume 1), Rev. E 49
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11.6.2.2 Cable requirements
11.6.2.2.1 Single ended I/O circuits ("W/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 imped-
ance match with cable terminators implies a cable characteristic impedance of 132 ohms (see 11.6.2.2.3).
Single ended I/O cable pin assignments are shown in Table 11.6.3-1a and 11.6.3-1c.
11.6.2.2.2 Differential I/O circuits ("WD/DC" models)
The maximum total cable length for use with drives having differential I/O drivers and receiver circuits shall be
25 metres (82 ft.). A stub length of no more than 0.2 metre (0.66 ft.) is allowed off the mainline interconnection
with any connected equipment. An ideal impedance match with cable terminators implies a cable character-
istic impedance of 122 ohms (see 11.6.2.2.3). Differential I/O pin assignments are shown in Tables 11.6.3-1b
and 11.6.3-1d.
11.6.2.2.3 General cable characteristics
In general, cables having the characteristic impedance's given in 11.6.2.2.1 and 11.6.2.2.2 are not available;
however, impedance's that are somewhat lower are satisfactory. A characteristic impedance of 100 ohm
±10% is recommended for unshielded flat or twisted pair ribbon cable. However, most available cables have
a somewhat lower characteristic impedance. To minimize discontinuances and signal reflections, cables of
different impedance's should not be used in the same bus. Implementations may require trade-offs in shield-
ing effectiveness, 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 main-
tain adequate margin at FAST SCSI transfer rates.