Teardown of a HomePlug Adapter


#1

So numerous people within the OpenROV project have mentioned to me that some flavor of Ethernet-over-Powerline (say, HomePlug AV or IEEE 1901) device represents the ultimate way to send data across the tether. They are fast (200 Mbit/sec for HomePlug AV and 500 Mbit/sec for IEEE1901), don't require an arbitrary cable impedance, and allow power to be sent over the tether simultaneously. Most importantly, they are inexpensive due to mass production- a pair of HomePlug adapters can be purchased for $40.

The drawback to date has been finding a solution that works without 110 VAC power. The off-the-shelf devices are meant to be plugged into a wall socket, which is fine if you want to send 110 VAC down the tether. But this has severe safety implications (I believe Eric has played with this, and has shocked himself numerous times), plus you would need to have an inverter at your laptop to generate 110 VAC for the tether, and then you would need to convert and rectify this power in the ROV to charge the batteries.

Just on the safety issues alone I would reject using off-the-shelf adapters.

Apparently somebody has looked into buying just the integrated-circuit chipset used in the adapters, but there was a large upfront fee involved, along with non-disclosure agreements and the like. A check for literature on the internet confirmed that you can't readily find datasheets for the part (typical vendors are Qualcomm, Broadcom, and Sigma Designs). So going this route seems a non-starter.

So I started wondering what it would take to cannibalize an off-the-shelf HomePlug adapter, and convert it to run on battery power while sending an arbitrary AC or DC voltage down the tether.

Sending an arbitrary voltage down the tether appears (so far) to be the easy part. Finding reference schematics on the internet is problematic, but it appears that the adapters use tranformer and capacitive coupling to isolate the HomePlug/Ethernet circuitry from the powerline. For instance, see the area marked "safety coupling" on this summary sheet:

http://www.qca.qualcomm.com/networking/technology.php?nav1=102&product=71

Given the isolation, we should be able to run pretty much any voltage we want, AC or DC, over the tether. The issue remaining is how to power the adapter- it's designed to take 110 volts off of the plug (now the tether) to power itself. If we could separate the HomePlug/Ethernet circuitry from the internal 110VAC-to-DC power supply, then we could power the adapter from a separate power supply such as USB.

Time for a teardown!

I purchased a pair of Medialink MHP-EA200 Powerline Network Adapters. They were cheap, $40 for the pair. It turns out, as you'll see below, that they are perfectly suited for being torn down.

Here's a picture of one of them. You can see that they're quite small, about 2.1"x2.1"x1.2", not counting the power outlet prongs:


The flat top of the adapter is surrounded by a gap that is sized right for inserting a medium flat-blade screwdriver:


With a bit of prying on 3 of the 4 sides of the flat top, the lid pops right off, revealing the circuitry guts:


Looking from the side, you can see that there are actually two separate circuit boards:


I smell victory already! When products like this are made with two separate boards, usually one is multi-layer with all the high-tech stuff on it, and the second one is a simple one- or two-layer board with the power supply and all the low-tech bits. If that's what this is, then we can just discard the power supply board and save the Ethernet board, if we can figure out how to power it. We just need to be sure that the isolation network ( the "safety coupling") is on the high-tech board that we're keeping.

So the boards remove easily from the plastic case, as the connections to the wall-socket prongs are wired rather than soldered directly to the circuit boards:


Note the two gold-colored pins connecting the two circuit boards. It looks like these pins carry the 110 VAC line to the board with the high-tech bits on it. Looking good!

Here's a view of the other side:


Note another 3 gold-plated pins bridging between the circuit cards. Perhaps these carry DC power from the power-supply board to the high-tech bits?

Here's the underside of the board stackup, after I desoldered the power supply prong connections:


You're looking at the underside of the power supply board. Note the fat traces and through-hole components.

Careful examination between the two boards showed that the five gold pins mentioned above were the only connections between the two boards. Some cutting with the smallest cutter I had separated the boards:


The big traces and big components on the power-supply board make it easy to trace what's going on. AC power comes in on the pads marked L (line?) and N (neutral?). The line connection is routed to a 10 ohm resistor, and after that there's an MOV across the power lines. These components seem to for surge protection. The power is then routed to the two gold pins that go to the high-tech board (marked J7), and to the nearby yellow transformer that steps-down the AC voltage for the power supply.

On the high-tech board, you can see the matching two pins are labeled J2. This is where we'll hook up the tether if everything works right. Note that just above J2 are two yellow capacitors and a transformer. This is the "safety coupling" that was referred to earlier. Victory! All of the networking bits-and-pieces appear to be on the one high-tech circuit board.

Here's another shot of the high-tech board:


J2, the tether connection, is on the lower left. At the top left you see the back side of the RJ-45 connector in black. Just to the right of that are the three pins that supply DC power to the board.

Upon close examination of the power supply board, I found that the two outer pins are connected together. So there is just a single voltage required to power this board. Plugging in only the power-supply section showed that it's a 3.3 volt supply, with ground on the outsides and +3.3V in the center. So, time to hook up just the high-tech bits to a lab power supply:


And that's the checkered flag! This is the HomePlug AV adaptor (200 Mbit/sec) with its power supply stripped away, running on an external 3.3 volt supply. Current draw is 0.26A (under 1 watt of power) in this idle state. I'm sure it will draw more once it acquires a network connection.

Next step- hook up a network over a piece of the tether cable.

-Walt


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#2

Very exciting! Excellent work and teardown writeup.


#3

Oh man! THIS IS GREAT! Haaaa! I was giggling to myself the entire time I read this article. Huge implications! I can't wait to see how it does on a network. Keep us posted!

Eric


#4

Awesome work! This might open up a whole new range of comm possibilities! Ill be keenly following your progress


#5

So I soldered some little connectors to both sides of a 15' piece of tether, and made up two connectorized power cords as well. Connected the tether to the two adapters, powered them off of two different lab power supplies, then connected one adaptor to my DSL router, and the other adapter to a laptop. Here's the result:


Current draw while operating is 0.35A at 3.3V, which is only a little over 1 watt.

I don't know how to measure the speed of the link, but given how slow my DSL is, it didn't seem to be any different than just using a patch cord.

Next step: For the laptop adapter, add a USB cable and a regulator to drop the +5V of USB down to 3.3V, and then package it back into the original case. For the ROV side, we'll have to figure out how to tap some 3.3V power off the BeagleBone.

Let's plan on doing some tests with the full tether during the upcoming build day.

-Walt


#6

great job!

i will follow this topic, in order to build my own tether like this i guess


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#7

Walt,

This is tremendous!

I just ordered another set of Medialink MHP-EA200 adapters which should be at OpenROV HQ before the build day on Saturday. This way we have one more set to tinker with.

I'm extremely excited about this- thank you!

Eric


#8

We should definitely try some tether tests, in water if possible, and if that is successful maybe try putting a little DC voltage across the tether and check that it doesn't interfere with anything. It will also be interesting to check whether the comm speed is affected by any EMI caused by operation of the motors. Is there a good software tool to measure link speed in real-time?

If all that looks good, I can walk you through a tear-down of the adapters you just bought. Now that I've got the hang of it it will take all of 5 minutes. We could maybe even videotape it for others who want to try this at home.

Do you have a small soldering station and solder at OpenROV HQ? I'm going to need to put some connectors on whatever length of tether you want to test on Saturday.

-W


#9

Walt,

That sounds great. We have access to water for testing and can introduce some EMI as well. OpenROV HQ has a temperature controlled soldering iron, ESD mat and numerous other gidgets and gadgets that might be useful for testing this out.

For examining network traffic and connection speeds, I would suggest using Wireshark (http://www.wireshark.org), a network protocol analyzer which seems very popular among many of my hacker-type friends. I am not yet very well versed in using it, but it doesn't seem like it would be very hard to learn.

If there is anything else that you think would be helpful for tinkering with this, let us know!

Eric


#10

Since it seems like we might want to play around with these adapters a bit at Saturday's Build Day, I ran up to Jameco today to pick up a bunch of the proper connectors to mate cleanly with the adapter boards. The correct part is a 1 row by 3 position female connector, to mate with 0.025" square posts on 0.100" centers. If you've ever examined the pins that are used on an Arduino shield, well, that's what the pins on the adapter are like. The Jameco part number for the connector housing is 157383; the insertable female pins are part #100766. I also picked up a crimper for the pins while I was there. It was only $12, and after crimping half a dozen pins, I wondered what the heck had ever taken me so long to shell out the bucks for the right tool. Here's a look at a finished connector:


The tether connection and the power connection both use the same 3-position connector. The tether uses the outside two pins. The power plug puts +3.3V on the center pin and ground on the outer two. Handy- you won't brick your adapter if you plug the power cord in backwards.

The next step was to build a USB +3.3V power supply, to power the topside adapter. I took a USB cable, lopped off one end, and peeled back some of the wires:


Thankfully the cable manufacturer was not devious- the black wire is ground and the red wire is +5V, as one would naturally assume. But I did check.

I build a little regulator circuit on a piece of perfboard to drop the USB +5V to +3.3V:


I used a National LM1117T-3.3 low-dropout regulator, coupled with a pair of 10 uF tantalum capacitors. I put a small heatsink on the regulator just to be safe, and added some strain relief for the USB cable because of the small power wires. This is a pretty good first prototype; the next one will be smaller to try to fit in the original case of the HomePlug adapter.

For the adapter that will be in the ROV, I worked a bit on the tether configuration to get it to fit into the electronics module better. If you plan on soldering the tether wires to the adapter board you have a nice compact configuration. But if you want to keep it connectorized for maintenance purposes, the upright position of the tether connector presents a challenge:


I decided to desolder the original pins and replace them with right-angle pins. Don't try this unless you are comfortable with desoldering multi-layer boards, or you don't mind taking the chance of bricking the adapter. Here's the result:


Now the tether comes cleanly out the side of the adapter:


Something similar might need to be done with the power supply connection, but it's in a tougher area of the board to work on, so I haven't decided what to do there yet.

So here's the result:


The laptop is connected to my DSL router over the tether as before, but now the laptop's Homeplug adapter is powered via USB. Bellissimo!

-W


#11

Stand alone Homeplug comm power FTW! We have a 50m and 100m tether here to test with, and I may try to pick up some (deliberately) low quality, random impedance twisted pair to play with too. Do you think transmitting over a higher voltage AC sine wave helps with propagation?

I'm REALLY looking forward to Saturday!

Eric


#12

No, I don't think the AC waveform makes any difference at all.

Right now I'm pondering how we're going to mount this thing in the ROV, specifically the ethernet connection to the BeagleBone. The shortest patch cable I have is 1 foot. There are some 6" ones available on-line, but unless I order them next-day air they won't be here by Saturday. I don't suppose OpenROV has a spool of Cat5 cable and a crimper for modular connectors? Maybe for now just curl the 1 foot patch cable up in a tight loop?

I've also got to figure out where to tap into the BeagleBone 3.3V. The BB has an auxiliary 1-amp 3.3V regulator that sends this power up onto the cape. It mostly seems to be used there for level translators, so I would hope that if we pulled another 350 mA on that rail we'll be OK. This 3.3V is brought out on pin 2 of connector J1 on the cape. The cape I have right now has nothing stuffed in the spot for J1. Can I assume that we can put some pins in there to drive the HomePlug adapter?

-W


#13

Yeah- I think that should be totally doable. Pin 2 on J1 would normally be generic 3.3v for an Arduino Shield devise (those header holes are arranged so that an Arduino Shield could be stacked on top of the cape) but since a shield won't fit on the cape when it's in the electronics tube, that pin won't be used otherwise. As far as super short patch cables, we've made some custom ones in the past and may have one or two lying around, but otherwise, we can just do the test with a longer patch cable.


#14

Sounds great. I will bring some header pins with me, to solder into the pads for J1, and will also bring the wire and connectors to make a custom power cable to go between the cape and the HomePlug adapter.

-W


#15

Sounds great! Again, let us know if there is anything else we should get from our end.

E


#16

Very impressive Walt!


#17

This is really great! A good idea. A good teardown. Some lucky breaks. And a good writeup.

It is what this group is all about.

SherpaDoug


#18

really nice find/guide! i'm definitly going to try out this route for my openrov!


#19

This adapter is sold in New Zealand (and I guess Australia) as a Tenda P200. I have a pair on my desk now, one in pieces and it's an identical device, just the two wires are soldered to a New Zealand style power plug.


#20

Nice. I suspect the electronic guts are a generic design, which is then sold to MediaLink or Tenda or whomever, who then packages it up, puts their name on it, and customizes the owner's manual and software disk for their market. The internal power supply board appears to be a generic switcher that can handle most anything you throw at it, 110V or 220V, 50 Hz or 60 Hz.

Just out of curiosity, how cheap were you able to find these Tenda P200's in New Zealand? The Medialink units are on sale right now from Amazon for $40/pair.

-Walt