Ethernet networks use 8-pin connectors casually called RJ45 (RJ stands for Registered Jack, and 45 refers to a particular interface standard). The term RJ45 refers to both modular jack and plugs, and often to the Ethernet cable itself. The Ethernet cable has eight wires grouped with four twisted pairs. Here we will provide general information on such cables.

RJ45 PINOUT AND COLOR CODE

The following diagram shows pinout of both RJ45 jack and the connector, as well as two common color codes of the wires (T568A and T568B).

RJ45 PIN DESIGNATIONS

A compliant PSE may provide power only via valid pairsets specified by IEEE802.3 standards (even the PoE systems that use some proprietary standards other than IEEE still use the same IEEE pinout). In each pairset, the two wires within a pair are under the same DC voltage and carry approximately the same current. The IEEE allows two pairset alternatives (modes): "Alt A" and "Alt B". At this, they allow different polarities. That's why PD devices always need input rectifier bridges. If the original standards specified only fixed polarity and the designers of the early systems waited until such standards ratified, there would be no need in the rectifiers. The industry then would save a lot of energy otherwise lost on the rectifiers. The following table provides pin designations defined by legacy IEEE802.3af and IEEE802.3at.



The following table provides pin designations defined by the latest IEEE802.3bt.



We see that IEEE802.3bt specification added alternative voltage polarity for mode B, which in my humble opinion was very unfortunate. They should've moved in the opposite direction by specifying fixed polarity for all modes. If they specified fixed polarity for all modes, it would allowed the designers to eliminate rectifier bridges in new systems, or at least to leave just one diode. Of course, without rectifier diodes the new systems would not be backward compatible with the exiting ones, but it could work for the specific systems when both PSE and PD have the same voltage polarity.