Figure 9.15: Common bus topologies: (a) point to point; (b) heavy point to point; (c)
daisy chain; (d) T topology; and (e) star topology.
1. Point to point. This is the simplest layout option. The major routing restrictions to worry
about are minimum and maximum line lengths and the ability to match the line
lengths in signal groups.
2. Heavy point to point. This topology has the same restrictions as above. Furthermore,
the short stubs that connect the main interconnect to the components should be kept
very short; otherwise, the topology will behave like a star or T-topology.
3. T topology. T topologies are usually unidirectional; that is, agent 1 (from
Figure 9.15d
)
is the only driver. As mentioned above, symmetry is necessary for adequate signal
integrity. The T topology is balanced from the view of agent 1 as long as the legs of
the T are the same lengths. Assuming that the legs of the T are equal in length, the
driver will only see an impedance discontinuity equal to one-half the characteristic
impedance of the transmission lines. If the T is not balanced (such as when agent 2
or 3 is driving), it will resonate and the signal integrity will be severely diminished. If
all three legs were made to be equal, the bus would become unidirectional because it
would be symmetrical from all agents (then the topology would be a three-agent star,
not a T). One trick that is sometimes used to improve T topologies is to make the T
legs twice the impedance of the base, which will eliminate the impedance
discontinuity at the junction (see
Section 4.6
).
4. Star topology. When routing a star topology, it is critical that the electrical delay of each
leg be identical; otherwise, the signal integrity will degrade quickly. Star topologies
are inherently unstable. It is also important to load each leg the same to avoid signal
integrity problems.
Summary :
Figure 9.15: Common bus topologies: (a) point to point; If all three legs were made to be equal, the bus would become unidirectional because it would be symmetrical from all agents (then the topology would be a three-agent star, not a T). One trick that is sometimes used to improve T topologies is to make the T legs twice the impedance of the base, which will eliminate the impedance discontinuity at the junction (see Section 4.6 ).
Tags :
topology,point,star,signal,impedance,topologies,integrity,legs,equal,lengths,agent,same,figure