am79c974 Advanced Micro Devices, am79c974 Datasheet - Page 130

• Manufacturer Part Number: am79c974
• Description: Pcnettm-scsi Combination Ethernet And Scsi Controller For Pci Systems
• Manufacturer: Advanced Micro Devices

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.
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AMD
P R E L I M I N A R Y
Am79C974
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
2. For a PCI CLK of 33 MHz, the maximum round trip
the ground plane or exclusively to one of the power
planes. If not, the power planes should be
decoupled.
time of any shared mother board signal should be
10 ns.