I have a board with two RJ45 connectors. One is used for an Ethernet port and the other one is for a RS485. The Ethernet is for Power over Ethernet. What are some suggestions to protect the RS485 port in case the operator connects the PoE at the RJ45 of the RS485 instead of the Ethernet one? Unfortunately, the PSE recognizes the RS485 port as a valid PD, and applies power to the RS485 transceiver. How can I avoid this? In case the operator is wrong to make the connection I would like to protect the RS485 transceiver and I would like the PSE not to supply power.

  • 4
    \$\begingroup\$ Change the RS485 to a different type of connector. Prevention, so they say, is better than cure. \$\endgroup\$
    – Andy aka
    Mar 25, 2021 at 13:47
  • \$\begingroup\$ Further to what Andy said - Poke Yoke - design the system so you can’t make a mistake. Otherwise Linear Tech have 60V tolerant RS485 transceivers and/or add tranzorbs and polyfuses. 48V might be a challenge thermally for the tranzorbs and the polyfuses. \$\endgroup\$
    – Kartman
    Mar 25, 2021 at 13:53
  • 1
    \$\begingroup\$ Change the RS485 connector pinout to something that does not interfere with PoE? Change termination to something that is not detected by PoE? \$\endgroup\$
    – Justme
    Mar 25, 2021 at 14:14
  • \$\begingroup\$ I proposed to the customer to change RS485 to a different type of connector but the customer does not want! He wants a protection circuit between the RS485 transceiver and the RJ45 connector \$\endgroup\$
    – Mark83
    Mar 25, 2021 at 14:27
  • \$\begingroup\$ Use channel protectors. Quite expensive but protect the lines even when they aren't powered. \$\endgroup\$ Mar 25, 2021 at 15:19

1 Answer 1



How can I avoid this?

Here is how I've dealt with this in the past.

  1. Color code the connectors or label the connectors (if possible)

  2. Use software to not turn on the POE unless the configuration is right.

  3. Different sized connectors (which you cant do, change the RJ45 to a RJ11)

Another option is somehow do some impedance detection and don't turn on the POE unless the right cable is detected. It shouldn't be hard to tell the difference between RS485 and Ethernet, one is a transformer and the other is a transceiver. I don't have your schematic so I can't recommend ways to do this directly.

Last option: If you can change the pinout of the RS485 (not sure what you're running but if your only running (Vcc D+ D- and GND) then you could rearrange the pins so they do not use the POE pins.


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