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82551QM/ER/IT Fast Ethernet PCI Controller MDI-X Functional Overview

Intel® 82551QM/ER/IT Fast Ethernet PCI Controller MDI-X (AP-472)

Introduction and Scope

This application note describes the Intel® 82551QM/ER/IT Fast Ethernet PCI Controller’s Management Data Interface MDI\MDI-X feature.

The MDI/MDI-X feature provides the ability to automatically detect the required cable connection type and configure the controller-side MAU to the cable type. This feature effectively allows all properly wired Ethernet cables, usable with any Ethernet device, to be connected to the 82551QM/ER/IT without any additional external logic.

This advanced feature enables auto-correction of incorrect cabling with respect to cross-over versus straight-through cables. The 82551QM/ER/IT can identify the cable connection type and adjust its MDI port to the cable by switching between the TD and RD pairs.


The auto-switching function is not supported when forcing speed and duplex modes. While in forced link mode, auto MDI-X cannot properly resolve auto-switching. Refer to Section 4.0 for the code that can be used to turn on/off auto MDI-X.

In a standard straight-through RJ-45 port configuration, the transmit pair is on contacts 1 and 2, and the receive pair on contacts 3 and 6. These are defined by Clause 23.7.1 of the IEEE 802.3u standard.

Read the full Intel® 82551QM/ER/IT Fast Ethernet PCI Controller MDI-X application note.

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