lxt9785 Intel Corporation, lxt9785 Datasheet - Page 158

• Manufacturer Part Number: lxt9785
• Description: Advanced 8-port 10/100 Mbps Phy Transceivers
• Manufacturer: Intel Corporation

LXT9785 and LXT9785E Advanced 8-Port 10/100 Mbps PHY Transceivers
4.13
4.13.1
4.13.2
4.13.2.1
4.13.2.2
160
Cable Diagnostics Overview
Debugging cable problems increases the overall cost of owning and operating a local area network.
Cable Diagnostic tools were incorporated into the LXT9785 device to help customers debug
network cable problems. The Cable Diagnostic tools provide the ability to detect severe cable
problems, such as open and short circuits, and determine the distance to the discontinuity.
Features
The following are three cases to consider for Cable Diagnostics:
An improperly terminated cable will not meet IEEE 802.3 return loss requirements. Register 29 has
been added to control cable testing and report cable testing results.
Cable Diagnostics provides a method to determine the distance to opens and shorts when the link
partner is inactive on the twisted-pair under test. The cable tests produce undefined results if the
link partner is transmitting signals. Implementation methods may vary depending upon the system
use requirements of Cable Diagnostics.
Operation
Cable Diagnostics utilizes the PHY transmit drivers and receivers to test a single twisted-pair. A
transmit pulse is driven down the twisted-pair under test and the reflected signal is analyzed. Link
partners transmitting NLP, FLP, MLT3, or other TDR pulses may interfere with the ability of the
LXT9785 to properly analyze the reflected Cable Diagnostic pulse. Implementation algorithms
must take these potential situations into consideration.
Short and Long Cable Testing Requirements
Implementing Cable Diagnostic tests, by enabling short and long cable tests sequentially, allows
more accurate measurements to a detected fault. Both tests are necessary to reach full precision.
The short and long cable tests can be run by writing 0x7400h and 0x6C00h to Register 29,
respectively. See
Precision
Cable Diagnostics estimates the distance to a fault up to 150 m. Category 5 or better cable produces
the most accurate test results. Less than Category 5 cable may produce less accurate results on long
cable lengths. Cable Diagnostics returns the distance to the closest fault, if a fault is present.
Cable Diagnostic tests report the distance to a cable fault based on the velocity of signal
propagation, which is used to determine the electrical length to the fault. The electrical length may
vary slightly from the physical cable length. The measurement accuracy may vary by +/- 2 m.
The following basic equation is used to calculate the distance to a fault:
•
•
•
Distance to a short circuit between wires of a single twisted-pair
An open circuit
Detection of an improperly terminated cable by the link partner.
Distance_to_Fault = (Reg29[7:0] - 3.5) / 1.16
Section 4.13.4, “Basic Implementation” on page 161
for implementation details.
Revision Date: August 28, 2003
Document Number: 249241
Revision Number: 007
Datasheet