am79c974 Advanced Micro Devices, am79c974 Datasheet - Page 123

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

SCSI System Considerations
INTRODUCTION
This appendix covers motherboard design considera-
tions which use the Am79C974. It discusses SCSI com-
ponent placement, signal routing, PCI interface
recommendations, noise considerations and termina-
tion schemes.
SIGNAL ROUTING AND SCSI
PLACEMENT
The main components for board design include the
Am79C974 (whose maximum trace length from the PCI
speedway is 1.5 ), SCSI connectors (either one or two
depending internal/external support), a 40 MHz crystal
oscillator and regulated terminators (on-board) which
can be up to .1 m (3.937 in) away from the Am79C974.
There are many ways to route SCSI bus traces on a host
adapter board or motherboard. Ideally, traces from the
Peripheral
External
APPENDIX E
High-density
Connectors
SCSI-2
High-Density
Termination
SCSI-2
bus cable
External
SCSI-2
Figure E-1. Ideal Routing Scheme
Am79C974
25
internal SCSI bus connector, from the Am79C974 and
from the external SCSI bus connector should all connect
in series. Care should be taken not to have any stubs in
the SCSI bus. (Stubs are any extensions off of the main
bus. The maximum SCSI bus stub length allowed is .1
m). This routing scheme helps maintain signal integrity
by reducing the possibility of signal reflections and other
undesirable effects. Auto-routing programs used for
board layout may not follow this scheme, and may cre-
ate “non-ideal” environments by routing internal and ex-
ternal on-board connectors first instead of routing both
sets of traces to the Am79C974. When peripherals are
added either internally or externally, a “three-pronged”
SCSI bus will be created instead of a linear one. If this or
any other stub problem occurs, changes should be
made manually to follow the ideal scheme. Refer to Fig-
ures E-1 and E-2.
connector for
internal peripheral
linear bus-
no stubs
Am79C974
to PCI Bus
Termination
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