Design Guide

I/O Subsystem

Intel

855GM/855GME Chipset Platform Design Guide 229

11.9.3.4. Critical Dimensions

There are two dimensions to consider during layout. Distance ‘A’ from the line RJ-45 connector to the

magnetics module and distance ‘B’ from the Intel 82562ET or Intel 82562EM to the magnetics module.

The combined total distances A and B must not exceed 4 inches (preferably, less than 2 inches). (See

Figure 116.)

Figure 116. Critical Dimensions for Component Placement

Intel

82562ET/EM

EEPROM

Magnetics

Module

Line

RJ45

Intel

ICH4-M

Distance Priority Guideline

A 1 < 1 inch

B 2 < 1 inch

11.9.3.4.1. Distance from Magnetics Module to RJ-45 (Distance A)

The distance A in Figure 116 above should be given the highest priority in board layout. The distance

between the magnetics module and the RJ-45 connector should be kept to less than one inch of

separation. The following trace characteristics are important and should be observed:

• Differential Impedance: The differential impedance should be 100 Ω. The single ended trace

impedance will be approximately 50 Ω; however, the differential impedance can also be affected by

the spacing between the traces.

• Trace Symmetry: Differential pairs (such as TDP and TDN) should be routed with consistent

separation and with exactly the same lengths and physical dimensions (for example, width).

Caution: Asymmetric and unequal length traces in the differential pairs contribute to common mode noise. This

can degrade the receive circuit’s performance and contribute to radiated emissions from the transmit

circuit. If the Intel 82562ET must be placed further than a couple of inches from the RJ-45 connector,

distance B can be sacrificed. Keeping the total distance between the Intel 82562ET and RJ-45 will as

short as possible should be a priority.