SNLS614A September 2018 – December 2018 DP83869HM
The DP83869HM uses Time Domain Reflectometry (TDR) to determine the quality of the cables, connectors, and terminations in addition to estimating the cable length. Some of the possible problems that can be diagnosed include opens, shorts, cable impedance mismatch, bad connectors, termination mismatches, cross faults, cross shorts, and any other discontinuities along the cable.
The DP83869HM transmits a test pulse of known amplitude down each of the two pairs of an attached cable. The transmitted signal continues down the cable and reflects from each cable imperfection, fault, bad connector, and from the end of the cable itself. After the pulse transmission, the DP83869HM measures the return time and amplitude of all these reflected pulses. This technique enables measuring the distance and magnitude (impedance) of non-terminated cables (open or short), discontinuities (bad connectors), improperly-terminated cables, and crossed pairs wires with ±1-m accuracy.
The DP83869HM also uses data averaging to reduce noise and improve accuracy. The DP83869HM can record up to five reflections within the tested pair. If more than 5 reflections are recorded, the DP83869HM saves the first 5 of them. If a cross fault is detected, the TDR saves the first location of the cross fault and up to 4 reflections in the tested channel. The DP83869HM TDR can measure cables beyond 100 m in length.
For all TDR measurements, the transformation between time of arrival and physical distance is done by the external host using minor computations (such as multiplication, addition, and lookup tables). The host must know the expected propagation delay of the cable, which depends, among other things, on the cable category (for example, CAT5, CAT5e, or CAT6).
TDR measurement is allowed in the DP83869HM in the following scenarios:
Software could read these registers at any time to apply post processing on the TDR results. This mode is designed for cases when the link is dropped due to cable disconnections. After a link failure, for instance, the line is quiet to allow a proper function of the TDR.