am79c974 Advanced Micro Devices, am79c974 Datasheet - Page 124

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

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 of switching the regulated
terminators on or off to accommodate peripheral con-
figurations. This approach also preserves the ideal
124
AMD
Peripheral
External
High-Density
Connectors
SCSI-2
High-Density
Termination
SCSI-2
External SCSI-2
Bus Cable
Figure E-2. A Poor Routing Scheme
P R E L I M I N A R Y
Am79C974
Bracketed card joining
ribbon cable with high-
density SCSI-2 cable
linear routing scheme. The following lists the require-
ments for implementation, while Figure E-3 illustrates
this approach.
(not recommended)
Connector for
Internal Peripheral
3-Pronged Bus
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.
to PCI Bus
25
Am79C974
stub
Termination
18681A-64