LAN8740AI-EN datasheet(48/136 Pages) MICROCHIP

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LAN8740A/LAN8740Ai

DS00001987A-page 48

 2013-2015 Microchip Technology Inc.

The TDR operates by transmitting pulses on the selected twisted pair within the Ethernet cable (TX in MDI mode, RX

in MDIX mode). If the pair being tested is open or shorted, the resulting impedance discontinuity results in a reflected

signal that can be detected by the LAN8740A/LAN8740Ai. The LAN8740A/LAN8740Ai measures the time between the

transmitted signal and received reflection and indicates the results in the TDR Channel Length field of the TDR Con-

trol/Status Register. The TDR Channel Length field indicates the “electrical” length of the cable, and can be multiplied

by the appropriate propagation constant in Table 3-9 to determine the approximate physical distance to the fault.

Since the TDR relies on the reflected signal of an improperly terminated cable, there are several factors that can affect

the accuracy of the physical length estimate. These include:

Cable Type (CAT 5, CAT5e, CAT6): The electrical length of each cable type is slightly different due to the twists-

per-meter of the internal signal pairs and differences in signal propagation speeds. If the cable type is known, the

length estimate can be calculated more accurately by using the propagation constant appropriate for the cable

type (see Table 3-9). In many real-world applications the cable type is unknown, or may be a mix of different cable

types and lengths. In this case, use the propagation constant for the “unknown” cable type.

TX and RX Pair: For each cable type, the EIA standards specify different twist rates (twists-per-meter) for each

signal pair within the Ethernet cable. This results in different measurements for the RX and TX pair.

Actual Cable Length: The difference between the estimated cable length and actual cable length grows as the

physical cable length increases, with the most accurate results at less than approximately 100 m.

Open/Short Case: The Open and Shorted cases will return different TDR Channel Length values (electrical

lengths) for the same physical distance to the fault. Compensation for this is achieved by using different propa-

gation constants to calculate the physical length of the cable.

For the Open case, the estimated distance to the fault can be calculated as follows:

Distance to Open fault in meters

TDR Channel Length * POPEN

Where: POPEN is the propagation constant selected from Table 3-9.

For the Shorted case, the estimated distance to the fault can be calculated as follows:

Distance to Open fault in meters

 TDR Channel Length * PSHORT

Where: PSHORT is the propagation constant selected from Table 3-9.

Note:

The TDR function is typically used when the link is inoperable. However, an active link will drop when oper-

ating the TDR.

TABLE 3-9:

TDR PROPAGATION CONSTANTS

TDR Propagation

Constant

Cable Type

Unknown

CAT 6

CAT 5E

CAT 5

POPEN

0.769

0.745

0.76

0.85

PSHORT

0.793

0.759

0.788

0.873

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