# Protect RJ45 CAN from POE

I'm trying to design a PCB with CAN Bus connection with STM32 (will be using a CAN transceiver) where we're using RJ45 for the CAN port. One of the main concern is what if the user plug in a Power Over Ethernet (POE) cable into the CAN port, will the CAN port be overvoltage and if so, how can I protect it from being overvoltage?

Edit: I don't need POE function in my design.

Thanks.

• Use only the pins which are used for the PoE pole (DC+ or DC-) or unused in the different PoE configurations. Better yet, use a different connector if it is a concern. Jan 24 at 17:28
• @EugeneSh. I don't need the POE function in my design. If there's no DC+ or DC- on my PCB, will the power still be transferred if the host from the other end has a power port and using a POE cable? Jan 24 at 17:36
• Isn't PoE isolated? What kind of PoE, the one that is off until negotiated, or the one that is forcibly on even without negotiation? Can you use Cat5 pairs so that you don't get PoE differential voltage at all? Jan 24 at 17:39
• Use a different connector for the CAN bus. Phoenix are sometimes used. Jan 24 at 17:44
• Then put a warning in the manual not to plug it to Ethernet like many other non-Ethernet devices with 8P8C connectors do. 8P8C connectors are already used for too many incompatible things. Jan 24 at 18:06

Note that "CAN bus" ISO 11898 doesn't specify any pin-outs, so the industry standard is instead set based on CANopen CiA 303-1.

Standardized RJ45 pin-outs:

CiA 303-1          PoE

1. CANH            TX+
2. CANL            TX-
3. CAN_GND         RX+
4. -               DC+ PoE
5. -               DC+ PoE
6. CAN_SHLD        RX-
7. CAN_GND         DC- PoE
8. CAN_V+          DC- PoE


Getting Rx+ on CAN_GND isn't ideal but hopefully won't break anything... maybe. You can skip connecting pin 3 on your CAN pin-out and only use pin 7.

There is an obvious issue in case you are using the CAN_V+ external supply for CAN, used to power galvanic isolation. Other than that, you shouldn't risk getting the DC+ signals from PoE into the CAN interface.

It might be wiser to use a different connector for CAN though. RJ10, M12 or DB9 etc.