Intel 386 manual Daisy Chaining

PHYSICAL DESIGN AND DEBUGGING

L=g"

Trace Is Microstrip

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Figure 11-18. Impedance Mismatch Example

Since Zs = 10 ohms, the termination techniques described previously will be needed to match the difference of 45.6 ohms. One method is to use a series terminating resistor of

45.6ohms or use A.C. termination where R=55.6 ohms and C=O.3 J.,LF. The terminated circuit of Figure 11-18 is shown in Figure 11-19.

11.4.2.2 DAISY CHAINING

In laying out PC boards, a stub or T-connection is another source of signal reflection. These types of connections act as inductive loads in the signal path. In daisy chaining, a single trace is run from the source, and the loads are distributed along this trace. This is shown in Figure 11-20.

An alternative way to this technique is to run multiple traces from the source to each load. Each trace will have unique reflections. These reflections are then transmitted down other traces when they return to the source. In such cases a separate termination is required for each branch. To eliminate these T-connections, high-frequency designs are routed as daisy chains.

Along the chain, each gate provides its own impedance load, thus it is, necessary to distribute these loads evenly along the length of the chain. Hence, the impedance along the chain will change in a series of steps and it is easier to match. The overall speed of this line is faster and predictable.

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Figure 11-19. Use of Series Termination to Avoid Impedance Mismatch

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