Ethernet, Over DC Power

We’re used to extending our network connections and being no longer constrained in our use of Ethernet by proximity to a switch or hub. Our houses routinely contain wireless networks, and of course powerline-Ethernet units passing data over our mains wiring. [Peter Franck] had a similar problem but without the mains power, for a distributed sprinkler system he needed to send Ethernet over DC cables.

The solution is a surprisingly simple one, taking one of those powerline Ethernet units and converting it by removing its mains power section. These devices contain the Ethernet and powerline modem chip with its associated circuitry, and a small switch-mode power supply. He’s removed the power supply and put in a capacitive coupling to the DC cabling, resulting in a relatively inexpensive DC powerline network device.

Powerline Ethernet devices are not without their own issues, for instance they are not popular with radio amateurs due to their effect on the RF noise floor. We’d therefore be curious to see what the RF emissions are like for this hack, but we still think it’s a useful weapon in the armoury as well as something to do with all those surplus powerline Ethernet bricks.

48 thoughts on “Ethernet, Over DC Power

    1. Where did you get the 30m from?

      To quote the source: “The purpose of this hack is powering and connecting multiple Raspberry Pi computers controlling a 24V DC solar irrigation system in a remote place with no AC or network line in reach and just 3 wires in the ground.”

      Possible reasons I see:
      1. W-LAN and water from an irrigation system isn’t the best combination.
      2. keep the air free from unnecessary low-usage networks. different networks for irrigation system (red), yourself (green) and guests (yellow/orange).
      3. better reliability
      4. PXE/etherboot/netboot

      1. To quote the source: “transmits 100 MBit/s over a thirty meters spool of telephone cable”

        Second point under “some data:” on Peter Franck’s site.

        Now obviously we don’t know if any/all of his runs were 30 meters but the point still stands that given those distances WiFi could have reached the devices. On the other hand there may not have been a good way to implement wifi.

        1. Well, the location where this is going to be deployed has topological difficulties that would render a WiFi system very costly. Having said that, there is a WiFi uplink to the internet from one of the nodes.

        2. BTW: the telephone cable was just meant to be an example lab setup. Actually, the deployment scenario is various lengths of 2.5 square mm power cables. 100 MBit/s is absolutely not necessary for the application. It just leaves a comfortable margin for signal attenuation in the field. I can add some more data when it will finally have been deployed.

          1. Please share more once you have details. I am quite curious how this will work. I would normally think of using RS485 for this, but then it gets complicated when you actually want internet access instead of just a serial link.

    1. Could only have been about 5 years before doing it all in 10/100 base T came down below total cost. Running 10 base 2 through the whole house was pretty decent even until media files got large.

      For giggles sometime I’m gonna try making baluns for 50 ohm to 300 ohm and try connecting vintage PCs with coax cards through Cat 5 runs.

      1. 10Mbit Ethernet ran just fine on phone line, and I don’t mean the twisted cat3 kind. Was it noisy? Yes. Did it allow 100 meter distance? Dunno. Never tried more than ~30, but guessing 100 would have been a stretch. 100Mbit over Cat3 was fine too in a pinch.

  1. It’s a nice idea, but for a sprinkler system does he really need 100MBit/s? I would guess some old RS232 would be enough for some command and status. IC is less than a dollar i guess, EMI is much better and it will work for hundreds of meters (depending on speed).

      1. I’ve seen this done before on HaD (maybe without the power injection) but what about the throughput? AFAIU WLAN and powerlan are both half-duplex but WLAN uses either 2,4 or 5GHz while HomeplugAV is between 2 and 68 MHz(!).
        The later seems far better suited for normal coax cabling (the ones used for DVB-T/S2).

        1. You just need decent enough cable, and it doesn’t even need to be THAT good. Losses over coax are linear with distance, they’re quadratic for free space propagation. Cabling always wins.

          Over 100 ft you can use pretty much anything you want RG-6 with F connectors is still only going to be like, 20-30 dB loss, which is the equivalent of free-space with 20 dBi gain antennas on both sides.

    1. Yes, that’s absolutely possible. Homeplug AV does not necessarily need power on the power wires. It has been proven by this hack.
      You can easily “UPS” the adapters by supplying 4.5 to 17 Volts to the modem circuitry but take extreme care when hacking them to stay on AC mains to avoid safety issues.

    2. A good idea, I wonder if the powerline devices would perform any better in this case too, with the noise reduced. Heck they might perform better without a blackout just by being powered from a more stable DC source..

      1. If you remove the switching power supply as described in the original posting it would in deed reduce the noise on your medium. However, the effect may be negligible since the original circuit provides inductors L2 and L4 as a low-pass filter between its internal power supply and the medium.

  2. This type of tech is actually being standardized as single pair Ethernet in IEEE and will be making it’s way into cars and industrial plants. Allegedly early tests even got it to work over two spools of barbed wire!

    1. Ok that’s just amazing and needs to be everywhere right now. I’d love to see 2.1mm plugs with this replace USB for Arduinos. If this ever makes it into an Espressif product it will be so useful!

  3. I did this once (happened as a side effect really). Hot a pair of AV200 tenda units, supposedly broken. Did some digging, supplied external 3V3. In the process, i figured i should test the connection without AC power between the units. Sure enough, it worked at full 100Mbit.

    Pretty nifty hack. Too bad there aren’t any consumer devices, that would use existing powerline chipsets but allow you to use any type of compatabile wiring you want.

    1. I chose the ones with a 100 MBit Ethernet interface for power consumption reasons. They were supposed to be the least power-hungry devices by spec. The modem is probably potentially faster than 100 MBit.
      You could choose the ones with a GBit Ethernet interface where power consumption is not an issue.

        1. Let me quote https://www.tp-link.com/us/home-networking/powerline/tl-pa4010/

          “Interface: 1*10/100Mbps Ethernet Port”

          So the limiting component is the Ethernet interface of this specific HP/AV adapters. Since I don’t even need 10 Mbps in my application I happily traded power consumption for speed.

          If you have a need for speed and you don’t care about an extra Watt, TL-PA7017 would be your best bet. Though I haven’t tried it out personally I would expect a very similar design except for the Ethernet interface part. At least from the outside they look very very similar.

          see https://www.tp-link.com/us/home-networking/powerline/tl-pa7017-kit/ for specifications

  4. If you used these on the common-mode center taps of PoE, could you do EoPoE? I’m actually curious if you could get gigabit speeds on the ethernet itself (despite the added common-mode noise) plus whatever speed you get from the powerline adapters, to increase total throughput.

    1. I would rather try multi-GBit interfaces to increase throughput in a scenario where structured cabling is already installed. I strongly doubt PLC will increase overall bandwidth on top of GBit ETH, with or without PoE, though I haven’t actually tried it.

    2. Your idea inspired this nefarious possibility:
      Could it be possible to establish a hidden/dark net through a PoE switch to jump VLANs?

      Basically connecting two compromised devices equipped with powline-LAN adapters on the common-mode center taps to two ports of a PoE switch on different VLANs.

      Depending on how the switch “does” its PoE it should be possible, shouldn’t it?

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