Acer SmartRAID V Bedienungsanleitung PDF herunterladen (Seite 199)

7-31

Theory of Operation

Beware of capacitance changes. As devices are added to a SCSI bus,

capacitance is introduced to each signal from the connectors,

receivers, and circuit board traces. The SCSI-3 working specification

limits this capacitance to 25pF because added capacitance lowers the

impedance of the cable near the device and adds delay. Look for input

filters that may be attached to the SCSI front-end of the device’s

printed circuit board. These filters add capacitance.

Avoid unnecessary connector converters. They cause impedance

discontinuity and signal reflections.

Route the cable with care. Avoid rolling the cable up on itself,

running the cable alongside of metal for long lengths or routing the

cable past noise generators (such as power supplies). Placing the

cable near ground planes created by grounded metal components also

reduces its impedance. The SCSI-3 draft specification suggests that to

minimize discontinuity due to local impedance variation, a flat cable

should be spaced at least 1.27 mm (0.050 in.) from other cables, any

other conductor, or the cable itself when the cable is folded.

Follow the SCSI specification for cable impedance. Cable

impedance for 10MHz SCSI systems should be limited to 84+/-12

Ohms (Ω). Cable impedance for 20MHz SCSI systems should be

limited to 90+/-12Ω for REQ and ACK signals and 90+/-10Ω for all

other signals.

Avoid mixing cable types. Select either flat or round, shielded or

unshielded. Typically mixing cables mixes impedance. Cable

impedance mismatch is a common problem that results in signal

reflections. Internal cables are normally unshielded flat ribbon cables,

while external cables should be shielded. Where they offer easier

routing, size advantages, and better air flow, round cables can also be

used internally. This may be beneficial if it allows for better impedance

matching to the external cable.

When round cable is used, select a cable that uses an optimal

placement of signals within the cable. Ribbon cable has good

crosstalk rejection due to the GND-Signal-GND layout. With standard

25-pair round cable, pairs are arranged in three layers. The closer a

pair is to the cable center, the higher the impedance. Using centrally

located high impedance pairs for speed-critical signals such as REQ

and ACK is a good choice. Locating the data pairs in the outermost

layer of the cable minimizes crosstalk between REQ, ACK and the

data lines. The middle layer might contain status lines such as C/D,

I/O, MSG, ATN, and so on. Make sure that the lowest impedance wire

in the cable is used for TERMPWR to minimize transmission line

effects on this voltage supply line. Some SCSI cables have a low-

impedance conductor specifically for this purpose.

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