Acer SmartRAID V Bedienungsanleitung PDF herunterladen (Seite 200)

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Theory of Operation

Cabling Ultra2 (LVD) SCSI

When the total length of a SCSI cable must exceed the maximum length

for single-ended buses, DPT recommends the use of an Ultra2 Low

Voltage Differential SCSI interface.

The following guidelines will help ensure reliability when configuring

Ultra2 SCSI systems:

• Use twisted-pair cable. Twisted-pair cable (either twisted-flat or

discrete wire twisted-pairs) provides greater signal integrity over

longer distances because noise coupled into a twisted-pair generally

appears equally on both wires. Because the differential receivers

respond to differences between the conductors of the twisted-pair, the

coupled common-mode noise is rejected.

Flat non-twisted ribbon cable can cause two problems. First, noise

introduced into parallel conductors tends not to be common mode.

Second, while the single-ended conductor arrangement interleaves

ground wires between signal wires, there are not enough conductors

to interleave grounds between each differential signal pair. These

factors lead to increased crosstalk between adjacent conductors on a

flat non-twisted ribbon cable.

• The maximum cable length for an LVD bus is 12 meters (39.3 feet).

• Because Ultra2 cables are sensitive to cable capacitance, the following

table of minimum spacing between devices should be followed:

Cable Capacitance Minimum Spacing

40 pF/m 36 cm

65 pF/m 22 cm

90 pF/m 16 cm

115 pF/m 13 cm

140 pF/m 10 cm

• Stub lengths should not exceed 0.1 meters (3.9 in.). The difference in

stub length should be less than 1.27 cm (0.05 in.) for the REQ, ACK,

DATA and PARITY signals.

• Terminators should be installed only at each physical end of the bus,

either on the last device attached or the unused connector at the end

of the cable. Ultra2 buses must be terminated with LVD terminators.

DPT cables are supplied with an LVD terminator installed at the end

of the cable.

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Acer SmartRAID V Bedienungsanleitung Seite 200