The DP83867 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 DP83867 transmits a test pulse of known amplitude (1 V or 2.5 V) 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 DP83867 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 DP83867 also uses data averaging to reduce noise and improve accuracy. The DP83867 can record up to five reflections within the tested pair. If more than 5 reflections are recorded, the DP83867 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 DP83867 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 DP83867 in the following scenarios:

• While Link partner is disconnected – cable is unplugged at the other side
• Link partner is connected but remains quiet (for example, in power-down mode)
• TDR could be automatically activated when the link fails or is dropped by setting bit 7 of register 0x0009 (CFG1). The results of the TDR run after the link fails are saved in the TDR registers.

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 dropped due to cable disconnections; after link failure, for instance, the line is quiet to allow a proper function of the TDR.