AM79C974 datasheet(130/153 Pages) AMD

130 / 153 page

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P R E L I M I N A R Y

130

Am79C974

Termination

There are three general termination schemes that apply

to motherboard or host adapter setups when using regu-

lated terminators. Each scheme recognizes that the

SCSI bus must be terminated on both ends. Therefore,

as more components and peripherals are added to the

bus, terminators must be relocated accordingly. Each

scheme is also based on an ideal routing situation, that

is one where the internal peripherals, external peripher-

als and the Am79C974 chip are connected by a linear

SCSI bus. See Figure E-1.

Scheme #1

In this case, the system uses only SCSI internal periph-

erals. The regulated terminators on board should be ac-

tivated. Peripherals can be added to the bus by

connecting them to a 50-pin ribbon cable. A SCSI termi-

nator should be attached to the last peripheral to termi-

nate the other end of the bus.

Scheme #2

In this case, the system uses only external SCSI periph-

erals. As in Scheme #1, the on-board regulated termina-

tors should be activated. In this scheme, a 50-pin high

density SCSI-2 cable should connect the external port

on the motherboard or host adapter to the first external

peripheral. Peripherals may be added with more cables

and the last peripheral should be terminated.

Scheme #3

In this case, both internal and external SCSI peripherals

are used. The regulated terminators should be deacti-

vated (since the Am79C974 will sit in the middle of the

SCSI bus). This may be accomplished through hard-

ware or software. The hardware approach involves de-

veloping a mechanism to detect peripherals connected

to the SCSI bus. This mechanism must then activate a

signal to turn the regulated terminators on or off accord-

ingly. The software approach involves having the user

tell the system interactively that the regulated termina-

tors should be turned on or off. Care should be taken to

ensure that each end of the Bus is terminated.

OTHER CONSIDERATIONS

The following are motherboard considerations for rout-

ing and layout. They should be taken into account along

with SCSI considerations.

Motherboards

1. High speed signals should be referenced only to

the ground plane or exclusively to one of the power

planes. If not, the power planes should be

decoupled.

2. For a PCI CLK of 33 MHz, the maximum round trip

time of any shared mother board signal should be

10 ns.

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