LAN8740A Datasheet, PDF (48/136 Pages) Microchip Technology – Small Footprint MII/RMII 10/100 Energy Efficient Ethernet Transceiver with HP Auto-MDIX and flexPWR® Technology

LAN8740A/LAN8740Ai
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.
Note: The TDR function is typically used when the link is inoperable. However, an active link will drop when oper-
ating the TDR.
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:
1. 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.
2. 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.
3. 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.
4. 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 * P_OPEN
Where: P_OPEN 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 * P_SHORT
Where: P_SHORT is the propagation constant selected from Table 3-9.
TABLE 3-9: TDR PROPAGATION CONSTANTS
TDR Propagation
Constant
P_OPEN
P_SHORT
Unknown
0.769
0.793
Cable Type
CAT 6
0.745
0.759
CAT 5E
0.76
0.788
CAT 5
0.85
0.873
DS00001987A-page 48
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