I'm somewhere in a process of creating mini automation system for hobby use.

I've already put CAT 5e cables with RJ-45 connectors in walls. They are somewhere between 5 - 30 meters long.


On one side of the cable there is a host device, on the other a few client devices connected in parallel.

I want to use differential communication (probably RS-485 or CAN) over one pair (A, B) and deliver power through the same cable.

I picked 24V DC as optimal voltage (reduced on devices to 5V by step-down converter). To reduce the resistance I want to use two wires for +24V and two wires for GND.

Now I'm facing a problem of choosing which RJ-45 pins should I use for those signals.

In my first prototype I assigned them:

1 - +24V
2 - +24V
3 - GND
4 - A
5 - B
6 - GND
7 - NC
8 - NC

During hot-plugging, there was a possibility that RS-485 / CAN could make contact before GND pins, but after +24V pins, causing the transceiver to blow.

I also want to make it more "not expirienced user" proof and protect both sides when device or host gets connected to Ethernet (possibly with PoE).

My second idea is to do this:

1 - A
2 - B
3 - NC
4 - GND
5 - GND
6 - NC
7 - +24V
8 - +24V

However on many solutions I have seen using 4 and 5 pin for data transmission is recommended. I have also seen numerous documents where 4 and 5 pin was left unused.

Which approach should I choose and why? Maybe ther?

  • \$\begingroup\$ Power over ethernet is already a standard with off the shelve devices for it, check it out. \$\endgroup\$ – ratchet freak Sep 30 '17 at 13:45
  • \$\begingroup\$ I don't think it's possible to use PoE with RS485 communication \$\endgroup\$ – peku33 Sep 30 '17 at 13:51
  • \$\begingroup\$ Then don't use RS485, ethernet capable microcontrollers are cheap enough. \$\endgroup\$ – ratchet freak Sep 30 '17 at 14:04
  • \$\begingroup\$ Depends on the speed of your ethernet connection, if you are using 100Mbit/s or lower, then will pin 4, 5, 7 and 8 be free to use. If you rather has a 1Gbit/s connection, then is all pins used for data and you have to use a PoE injector. \$\endgroup\$ – BufferOverflow Sep 30 '17 at 14:07

enter image description here

Have you considered the POE standard PHY approach?

More options

  • 2
    \$\begingroup\$ Of course I did. But I think this is overkill for devices such as temperature sensors. \$\endgroup\$ – peku33 Sep 30 '17 at 13:45
  • \$\begingroup\$ A PHY is necessary with a CM choke to attenuate large CM E fields from power cables. Don't overlook this. Controlled impedance from sensor to receiver is essential with ESD and Vcc clamp protection to data ports. \$\endgroup\$ – Tony Stewart EE75 Sep 30 '17 at 13:46
  • 1
    \$\begingroup\$ Have you defined all the requirements of your design yet in the question before you ask pick a pin config? no. \$\endgroup\$ – Tony Stewart EE75 Sep 30 '17 at 16:34

If you’re interested in using CAN, there’s a standard RJ45 wiring:

enter image description here


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