Product specifications

Design Guide - VT82C694X Apollo Pro133 with VT82C686A
Preliminary Revision 0.5, November 19, 1999 40 Motherboard Design Guidelines
Technologies, Inc.
We ConnectWe Connect
2.3.3 Routing Styles and Topology
High-speed bus signals are sensitive to transmission line stubs, which can result in ringing on the rising edge caused by the high
impedance of the output buffer in the high state. In order to maintain better signal quality, transmission stubs should be kept under
1.5 inches. Therefore, daisy chain style routing is strongly recommended for these signals. Figure 2-30 below shows an example
of a daisy chain routing.
ASIC
or
Connector
ASIC ASIC
Short Stub
Trace Length
ASIC
or
Connector
Figure 2-30. Daisy Chain Routing Example
Topology is the physical connectivity of a net or a group of nets. Basically, there are two types of topologies for a motherboard
layout: point-to-point and multi-drop. An example of these topologies is shown in Figure 2-31.
ASIC
or
Connector
ASIC
ASIC
Multi-Drop
ASIC
or
Connector
Point-to-Point
Figure 2-31. Point-to-Point and Multi-Drop Topology Examples
If daisy chain routing is not allowed in some circumstances, different routings may be considered. An alternative topology is
shown in Figure 2-32. The branch point in this case is somewhere between both ends. It may be near the source or near the loads.
Being close to the load side is best. The separated traces should be equal length.
ASIC
ASIC
or
Connector
Equal Length
ASIC
or
Connector
Somewhere in
the middle
Figure 2-32. Alternate Multi-Drop Topology Example