• 4,5 +
• 7,8 -

Cheap passive PoE injectors probably use pairs (4,5) and (7,8) exclusively for power, and disconnect them from data, so probably will only work at 100 Mbps.

I bought some cheap PoE injector/splitters from ebay. They claim gigabit speed, and a quick test shows they negotiate at that.

Pins 1,2,3,6 are directly connected through. Pins 4,5,7,8 seems to be connected with a serial DC blocking cap of what my multimeter says is ~93nF.

Cheap component testers says:

• 0.41mH power to combined side pin 4.
• 0.73mH between combine side pins 4 and 5.

It seems to be a bias T! I was expecting transformers with central tap, as is used on the ends normally.

Four pins, each connects to centre tap of each pair's transformer.

• + (1,2)
• - (3,6)

Only needs two pairs for 100 Mbps and power.