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:
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.