KS8893ML datasheet(27/106 Pages) MICREL

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Micrel

KS8893M/ML/MI

June 2005

M9999-063005

LinkMD Cable Diagnostics

The LinkMD feature utilizes time domain reflectometry (TDR) to analyze the cabling plant for common cabling

problems such as open circuits, short circuits and impedance mismatches.

LinkMD works by sending a pulse of known amplitude and duration down the MDI and MDI-X pairs and then

analyzes the shape of the reflected signal. Timing the pulse duration gives an indication of the distance to the

cabling fault with maximum distance of 200m and accuracy of +/- 2m. Internal circuitry displays the TDR

information in a user-readable digital format.

Note: Cable diagnostics are only valid for copper connections and do not support fiber optic operation.

Access

LinkMD is initiated by accessing registers {26,27} and {42,43}, the LinkMD Control/Status registers, for ports 1

and 2, respectively; and in conjunction with registers 29 and 45, Port Control Register 13, for ports 1 and 2,

respectively.

Alternatively, the MIIM PHY registers 0 and 29 can be used for LinkMD access.

Usage

The following is a sample procedure for using LinkMD with registers {26,27,29} on port 1.

1. Disable auto MDI/MDI-X by writing a ‘1’ to register 29, bit [2] to enable manual control over the differential pair

used to transmit the LinkMD pulse.

2. Start cable diagnostic test by writing a ‘1’ to register 26, bit [4]. This enable bit is self-clearing.

3. Wait (poll) for register 26, bit [4] to return a ‘0’, indicating cable diagnostic test is completed.

4. Read cable diagnostic test results in register 26, bits [6:5]. The results are as follows:

00 = normal condition (valid test)

01 = open condition detected in cable (valid test)

10 = short condition detected in cable (valid test)

11 = cable diagnostic test failed (invalid test)

The ‘11’ case, invalid test, occurs when the KS8893M is unable to shut down the link partner. In this instance,

the test is not run, since it would be impossible for the KS8893M to determine if the detected signal is a

reflection of the signal generated or a signal from another source.

5. Get distance to fault by concatenating register 26, bit [0] and register 27, bits [7:0]; and multiplying the result

by a constant of 0.4. The distance to the cable fault can be determined by the following formula:

D (distance to cable fault) = 0.4 x {(register 26, bit [0]),(register 27, bits [7:0])}

D (distance to cable fault) is expressed in meters.

Concatenated value of registers 26 and 27 is converted to decimal before multiplying by 0.4.

The constant (0.4) may be calibrated for different cabling conditions, including cables with a velocity of

propagation that varies significantly from the norm.

For port 2 and for the MIIM PHY registers, LinkMD usage is similar.

KS8893ML Datasheet(PDF) 27 Page - Micrel Semiconductor