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MCU is usually connected to a PoE controller via I2C interface. Most of the relevant documents show that I2C should be optically isolated in this application. What are the reasons to have such isolation?

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The PoE side is an isolated module that you want to keep separate from your internal logic.

There are a number of reasons why, but most importantly.

  1. You do not want pass logic noise from your digital side to the Ethernet side.

If the PoE were connected to your logic ground you may pass significant noise out onto the Ethernet cables disrupting transmission bandwidth.

  1. You do not want extraneous voltage events that may occur in the Ethernet cables propagating to your logic side.

  2. When your logic side is turned off the PoE side wont be. Isolation helps prevent the PoE from trying to power up your logic through IO pins.

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  • \$\begingroup\$ Thank you for the great reply! Could the first reason really increase BER on the Ethernet interface? \$\endgroup\$
    – Konstantin
    Apr 6 '17 at 16:04
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    \$\begingroup\$ @Konstantin, thanks. Yes, if your system has a lot of switching noise it can easily be coupled back into the Ethernet cable. \$\endgroup\$
    – Trevor_G
    Apr 6 '17 at 16:15
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One of the biggest reasons is for safety.

Since the beginning of networking there has been a requirement to isolate the electronics of a PC or electronic module from a fault that could occur on another PC or in the building infrastructure.

If for example another PC on the network developed a power supply fault that caused it to be live it could conduct along the network to make an electrocution hazard to other users. To avoid this the networking standards have a requirement for about 1000v of isolation between the wires of the incoming network and the rest of the module. For the data transmission this is normally accomplished by using small transformers.

PoE introduced another level of complication - the power is usually passed through transformers but where control signals need to be communicated across the boundary opto-isolators can be used.

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