AM79C974 datasheet(126/153 Pages) AMD

126 / 153 page

AMD

P R E L I M I N A R Y

126

Am79C974

18681A-66

This is optional. Use

accordingly to terminate both

ends of the SCSI Bus.

CPU

Memory

Internal

Drive

(Terminated)

Riser Card

High-Density

SCSI-2

Termination

External SCSI

Bus Cable

External

Peripheral

High-Density

SCSI-2

Connectors

Keyboard

Connector

To SCSI CLK1

and CLK2

Am79C974

To DC Power

Ferrite Bead

to PCI Bus

Mounted

High-Density

SCSI-2

Connector

Figure E-4. Motherboard Layout — Approach #2

Motherboard designs which place an internal and exter-

nal connector on either side of the Am79C974 are dis-

couraged since:

Two SCSI connectors are required on the mothe-

rboard—one more than what the other two ap-

proaches call for.

When the number of internal and external bus

components increase, the on-board terminators

would have to be turned off either through software

or hardware, which may be undesirable to a board

designer.

The possibility for creating stubs exists if the con-

nectors and Am79C974 are not routed correctly.

See Figure E-1 for the ideal routing scheme.

NOISE CONSIDERATIONS

Some areas of a PCI motherboard or host adapter de-

sign (which include the Am79C974) are more suscepti-

ble to noise. They are:

the 40 MHz crystal oscillator

the SCSI cables

the DC power planes

pins on the ICs, specifically the Am79C974.

Electromagnetic Interference (EMI)

There are several ways to reduce the amount of noise

present in these areas:

A 40 MHz crystal oscillator must be used in order

to have a 10 MB/s SCSI data rate. Use of this

40 MHz crystal oscillator, which drives the

SCSI CLK pin on the Am79C974, introduces the

possibility of unwanted high frequency compo-

nents, including harmonics, coupling with

Am79C974 signals. To help prevent this, a resistor

should be placed in series with the oscillator to

form a low pass filter with the input capacitance

(10 pF) of the SCSI CLK pin. This filter reduces the

edge rate of the clock waveform, increasing both

rise and fall times by 2 ns. This removes higher

frequencies, specifically harmonics from the crystal

oscillator waveform and thus reduces the amount

of noise introduced into the Am79C974.

A ferrite bead, which is essentially an inductor,

may be used with the oscillator to prevent coupling

of higher frequencies with DC power supply sig-

nals. The ferrite bead blocks high frequency noise

while acting like a short to the DC components.

AM79C974 Datasheet(PDF) 126 Page - Advanced Micro Devices

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