88e3015 Marvell Semiconductor, Inc., 88e3015 Datasheet - Page 39

• Manufacturer Part Number: 88e3015
• Description: Integrated 10/100 Fast Ethernet Transceiver
• Manufacturer: Marvell Semiconductor, Inc.

2.9 Virtual Cable Tester
The 88E3015/88E3018 devices Virtual Cable Tester (VCT™) feature uses Time Domain Reflectometry (TDR) to
determine the quality of the cables, connectors, and terminations. Some of the possible problems that can be
diagnosed include opens, shorts, cable impedance mismatch, bad connectors, termination mismatch, and bad
magnetics.
The 88E3015/88E3018 devices transmit a signal of known amplitude (+1V) down each of the two pairs of an
attached cable. It will conduct the cable diagnostic test on each pair, testing the TX and RX pairs sequentially. The
transmitted signal will continue down the cable until it reflects off of a cable imperfection. The magnitude of the
reflection and the time it takes for the reflection to come back are shown in the VCT registers 26.12:8, 26.7:0,
27.12:8, and 27.7:0 respectively.
Using the information from the VCT Registers 26 and 27, the distance to the problem location and the type of
problem can be determined. For example, the time it takes for the reflection to come back, can be converted to
distance using the cable fault distance trend line tables in
together with the distance will indicate the type of discontinuity. For example, a +1V reflection will indicate an open
close to the PHY and a -1V reflection will indicate a short close to the PHY.
When the cable diagnostic feature is activated by setting Register 26.15 bit to one, a pre-determined amount of
time elapses before a test pulse is transmitted. This is to ensure that the link partner loses link, so that it stops
sending 100BASE-TX idles or 10 Mbit data packets. This is necessary to be able to perform the TDR test. The
TDR test can be performed either when there is no link partner or when the link partner is Auto-Negotiating or
sending 10 Mbit idle link pulses. If the 88E3015/88E3018 devices receive a continuous signal for 125 ms, it will
declare test failure because it cannot start the TDR test. In the test fail case, the received data is not valid. The
results of the test are also summarized in Register 26.14:13 and 27.14:13.
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The definition for shorts and opens is arbitrary and the user can define it anyway they desire using the information
in the VCT registers. The impedance mismatch at the location of the discontinuity could also be calculated know-
ing the magnitude of the reflection. Refer to the App Note "Virtual Cable Tester
details.
Figure 11: Cable Fault Distance Trend Line
Copyright © 2006 Marvell
October 26, 2006, Advance
11 = Test fail (The TDR test could not be run for reasons explained above)
00 = Valid test, normal cable (no short or open in cable)
10 = Valid test, open in cable (Impedance > 333 ohms)
01 = Valid test, short in cable (Impedance < 33 ohms)
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Document Classification: Proprietary Information
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CONFIDENTIAL
®
re g26[7:0], re g27[7:0]
Feature
100
TX/RX
y = 0.7861x - 18.862
Figure
150
11. The polarity and magnitude of the reflection
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®
-- How to use TDR results" for
Virtual Cable Tester® Feature
Doc. No. MV-S103657-00, Rev. C
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