Power back-off mode and circuit for 100BaseT

Generally, 100BaseT allows for the establishment of links on cables (such as Category 5 or CAT5 cables) up to 120 m or more in length. In a given industrial Ethernet system, many of the cables deployed will be less than 120 m in length, and, in fact, many of the cables are much shorter than 120 m. Yet, the transmission amplitude output from each unit is generally the same, regardless of cable length. Here, Power Back Off (PBO) circuitry is provided in a unit that operates in a PBO mode to passively estimate a cable length and adjust its corresponding transmission amplitudes to generally match the estimated cable length so as to reduce power consumption.