Seagate ST12450W, Руководство по эксплуатации продукта

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54 ST12450W/WD Product Manual, Rev. A
11.7.1.2.1 Single-ended I/O circuits
The maximum total cable length for use with drives having single-ended I/O
driver and receiver circuits is 6 meters (19.7 feet) when operating at line data
transfer rates of 5 Mbytes/second or less, and 3 meters (9.85 feet) when
operating at transfer rates greater than 5 Mbytes/second (fast SCSI). A stub
length of no more than 0.1 meter (0.33 feet) 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 for ST12450W drives are shown in
Table 10.
11.7.1.2.2 Differential I/O circuits
The maximum total cable length for use with drives having differential I/O
drivers and receiver circuits is 25 meters (82 feet). A stub length of no more
than 0.2 meter (0.66 foot) is allowed off the mainline interconnection with any
connected equipment. An ideal impedance match with cable terminators
implies a cable characteristic impedance of 122 ohms. Differential I/O cable
pin assignments for ST12450WD drives are shown in Table 11.
11.7.1.2.3 General cable characteristics
In general, cables having the characteristic impedances given in Sections
11.7.1.2.1 and 11.7.1.2.2 are not available; however, impedances that are
lower are satisfactory. A characteristic impedance of 100 ohm
+
10% is
recommended for unshielded flat or twisted-pair ribbon cable. To minimize
discontinuities and signal reflections, cables of different impedances should
not be used in the same bus. Your specific setup may require tradeoffs in
shielding 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 to maintain adequate margin at fast SCSI transfer rates.
11.7.2 Connector requirements
Recommended mating wide cable connectors part numbers:
AMP 786096-7, female 68-pin
AMP 786090-7, male 68-pin
11.7.3 Electrical description
ST12450W drives use single-ended interface signals. These signals must be
terminated with 110 ohm active terminator circuits at each end of the total
cable. Single-ended circuits use open collector or three-state drivers.
ST12450W drives can be configured to provide the SCSI termination.
ST12450WD drives use differential interface signals. Each of these signals
must be terminated at each end of the total cable with 330 ohms to
+
5V and
330 ohms to ground with 150 ohms between each differential pair. All I/O
circuits are open collector, three-state drivers. Differential I/O drives are
shipped without terminators and have no provisions for adding terminator
sockets on the PCB. You must provide external termination for these drives.