AM79C974 datasheet(124/153 Pages) AMD

124 / 153 page

AMD

P R E L I M I N A R Y

124

Am79C974

18681A-64

External SCSI-2

Bus Cable

External

Peripheral

High-Density

SCSI-2

Connectors

High-Density

SCSI-2

Termination

Am79C974

to PCI Bus

Connector for

Internal Peripheral

Termination

3-Pronged Bus

(not recommended)

stub

Bracketed card joining

ribbon cable with high-

density SCSI-2 cable

Figure E-2. A Poor Routing Scheme

The Motherboard

The following two layouts may be used as a guideline

for the design of motherboards which incorporate the

Am79C974. For both layouts, the SCSI connectors

should be placed as far away from the PCI Speedway as

possible. Each layout refers to termination considera-

tions which are described in further detail in the Termi-

nation Considerations section.

Layout #1

This approach avoids the cost of placing an external

connector on the motherboard. In this configuration, the

Am79C974 is always at one end of the SCSI bus, there-

fore, the regulated terminators remain active. This elimi-

nates

the

problem

switching

the

regulated

terminators on or off to accommodate peripheral con-

figurations. This approach also preserves the ideal

linear routing scheme. The following lists the require-

ments for implementation, while Figure E-3 illustrates

this approach.

One connector on the motherboard connected to

one end of the internal bus ribbon cable and its

components.

The other end of the internal bus ribbon cable con-

nected to one end of the external bus high density

cable and its peripherals via a bracketed add-on

card. The internal bus may also be crimped to a

SCSI connector mounted on a bracket.

On board regulated terminators a maximum of

0.1 m (3.937 in) from the Am79C974.

External terminators connected to the end of the

external bus.

AM79C974 Datasheet(PDF) 124 Page - Advanced Micro Devices

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