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Interfacing Intel® 8255x Fast Ethernet Controllers Without Magnetics


The goal of this document is to enable the construction of an interface for the 8255x without the use of the 1:1 transformer previously required. This reduces cost and size by eliminating the transformer and replacing it with low-cost passive components such as resistors and capacitors.

This docu...ment, and its associated circuitry, was conceived and developed in response to many requests by our OEM customers involved in embedded designs to reduce component expense and “real estate” use on their boards. These new designs eliminate the need for the relatively large and expensive magnetics module commonly used between PHYs and the RJ45 connectors as well as in backplane and other such direct connect applications. We have developed a simple solution that can be used in a wide variety of such applications with the intent of simplifying the design cycle and reducing development time enabling products to enter the market in a shorter time frame than otherwise might be possible.

Since different LAN devices use different DC-biasing and different current source circuits, there is no single, magnetic-less circuit that will allow the LAN to work properly with every different, possible pair of LAN components. Magnetic-less LAN designs with good networking performance and extremely good Bit Error Rates (BERs), will not necessarily be fully IEEE compliant. Typically, they should comply with many, but not necessarily all, of the IEEE specifications.


Magnetic-less LAN designs should not be done when the LAN signals must be routed through cables that are external to the system chassis. The isolation transformer in a magnetics module provides some level of improved safety in the event that higher voltages or ESD gets onto the LAN cable. Magnetic-less LAN designs should only be done when the differential circuits or cables will be routed internal to the same chassis.

Read the full Interfacing Intel® 8255x Fast Ethernet Controllers Without Magnetics Application Note

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