WBLXT9785HE.D0-865114 Cortina Systems Inc, WBLXT9785HE.D0-865114 Datasheet - Page 157

• Manufacturer Part Number: WBLXT9785HE.D0-865114
• Manufacturer: Cortina Systems Inc

Specifications of WBLXT9785HE.D0-865114

• Lead Free Status / RoHS Status: Compliant

LXT9785/LXT9785E
Datasheet
249241, Revision 11.0
16 April 2007
4.13.2
4.13.2.1
4.13.2.2
4.13.3
Cortina Systems
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/LXT9785E 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
page 158
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.
Implementation Considerations
Before performing Cable Diagnostics, the twisted-pair to be tested may be verified to be
inactive. All applicable link configurations should be attempted. Cable Diagnostic tests
may be started if the attempts indicate no link partners are active. If link partners are
detected, additional tests and decisions as to next steps may need to be implemented in
the cable testing algorithm to ensure the most accurate results.
Cortina recommends that a 100BASE-TX link be attempted with MDI and MDIX enabled
sequentially, prior to performing Cable Diagnostic testing, to determine if a 100BASE-TX-
only link partner is present. If a link partner is in forced 100BASE-TX operation,
transmitting MLT3, the Cable Diagnostic test result will be undefined due to the
interference MLT3 causes in attempting to process the reflected Cable Diagnostic pulse.
Auto MDI/MDIX on the link partner should be accounted for in deriving the cable testing
algorithm.
Cortina recommends auto MDI/MDIX be disabled when running the cable tests. The
transmit and receive twisted-pairs must be tested one at a time with both short and long
cable test suites. The MDI/MDIX control bits in
27, Hex 1B), on page 207
requirement creates a minimum of four test permutations that must be completed to
determine if the fault exists, the distance to the fault.
®
LXT9785 and LXT9785E Advanced 8-Port 10/100 Mbps PHY Transceivers
Distance_to_Fault = (Reg29[7:0] - 3.5) / 1.16
for implementation details.
The following basic equation is used to calculate the distance to a fault:
can be used to select the twisted-pair to be tested. This
Section 4.13.4, Basic Implementation, on
Table 100, Trim Enable Register (Address
4.13 Cable Diagnostics
Overview
Page 157