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I am asking this because after doing some research I found some conflicting information.

First, on Wikipedia page on Ethernet physical layer I found that the voltage on the lines is 2.5V. https://en.wikipedia.org/wiki/Ethernet_physical_layer

But in more trainings and course that I take in Uni the professors said that it is 9V or 12 V.

Are there more standards for the physical connections and voltages on ethernet lines?

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There's many different voltage, and it varies depending of frequencies

From the IEEE 802.3-2008 document borrowed from the official IEE 802.3 get page - but it seems it is no more freely available.

7.4.1.3 AC common-mode output voltage
The magnitude of the ac component of the common-mode output voltage of the driver, measured between the midpoint of a test load consisting of a pair of matched 39 > Ω ± 1% resistors and circuit VC, as shown in Figure 7–13, shall not exceed 2.5 V peak from 30 Hz to 40 kHz and 160 mV peak from 40 kHz to BR.

7.4.1.4 Differential output voltage, open circuit
The differential output voltage into an open circuit, measured at the interface connector of the driving unit, shall not exceed 13 V peak.

7.4.1.5 DC common-mode output voltage
The magnitude of the dc component of the common-mode output voltage of the driver, measured between the midpoint of a test load consisting of a pair of matched 39 Ω ± 1% resistors and circuit VC, as shown in Figure 7–13, shall not exceed 5.5 V.

Newer standards may have different limits, but since they are usually retro-compatible, they must not have changed much. I dug trough a few of them but didn't found specifications that contradict those above.

The 802.3bq-2016: Physical Layer and Management Parameters for 25 Gb/s and 40 Gb/s Operation, Types 25GBASE-T and 40GBASE-T document has the table:

802.3bq2016 table 113A-2

in which you can find a Differential-mode voltage of < 2.4 + 19.68 (f / 30) mVpp close enough to 2.5V. (Vpp stands for the voltage differential peak-to-peak, i.e. difference between the lowest and highest voltage in a period.)

There's another voltage involved, when using Power Over Ethernet (POE), which range from 37 to 57V.

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