Applies to: All Power over Ethernet devices

Summary:

This article will cover the pinouts of different POE modes. Older, slower networks use Mode A, while newer network cabling uses Mode B. We will also review 802.3bt type 3 and 4 pinouts for units receiving more than 30 watts.  

Summary:

• Two modes of PoE exist, A and B.
• Mode A delivers power on the data pairs of 100BASE-TX or 10BASE-T.
• In mode A, pins 1 and 2 form one side of the 48 VDC, and pins 3 and 6 form the other.
• These are the same two pairs used for data transmission in 10BASE-T and 100BASE-TX, allowing the provision of power and data over only two pairs in such networks.
• The free polarity allows PoE to accommodate crossover cables, patch cables, and auto-MDIX.
  
  
  
  
• Mode B delivers power to the spare pairs. PoE can also be used on 1000BASE-T Ethernet, in which case there are no spare pairs, and all power is delivered using the phantom technique.
• In mode B, pins 4–5 form one side of the DC supply, and pins 7–8 provide the return; these are the "spare" pairs in 10BASE-T and 100BASE-TX. Therefore, mode B requires a 4-pair cable.
  
  
  
  
• A phantom power technique also allows the powered pairs to carry data. This permits its use not only with 10BASE-T and 100BASE-TX, which use only two of the four pairs in the cable, but also with 1000BASE-T (gigabit Ethernet), 2.5GBASE-T, 5GBASE-T, and 10GBASE-T which use all four pairs for data transmission.

• 802.3bt PoE Mode A. In mode A, pins 1-2 and 3-6 are used to send and receive data and energy. The pins 4-5 and 7-8 remain unused. Equipment that uses mode A is commonly known as end-Span.

• 802.3bt PoE Mode B. In the B mode, no pin pair is left unused. Mode B uses pins 1-2 and 3-6 to send data, and the power flow is transmitted through pins 4-5 and 7-8. Pins 4-5 and 7-8 are sometimes called spare pairs or phantom pairs, and the devices that use mode B are known as Midspan.