How does Tether communication works?


#1

I am wondering how fundamentally the tether communication works?
When we press the key (W) on keyboard, the port and starboard motors start to run.
what kind of control signal is generated and pass over the tether cable from the Topside adapter to the homeplug adapter on the ROV?

It is definitely not sending different DC voltages as I measure the voltage across the two tether cables, it gives me a constant voltage of 4.4~4.5V. So it could be a small AC signal? As I cannot visualize any changes in terms of AC voltage using a multi-meter.

Is this some sort of power line communication stuff?

Could you explain in both microscopic (such as EM waves, Current, RF and etc) and macroscopic (such as coding, architecture) point of view?

The cat5 cable has 4 wires (Transmit+/- and Receive+/-) and the network has only two wire (Tether A and B). what kind of signal is running through the tether cable? Is it some sort of electrical pulses? Any waveform diagrams? I really would like to imagine what do those network signals look like?

Could you show me the difference between the network signals for turning the internal LED on and turning the motor on? each key has an associated plugin, but what do those plugins do in terms of signal generation?


#2

It communicates using Ethernet over power. So it’s RF modulated on a DC voltage. Much too quick to pick up with a meter.

The BBB in the ROV runs a webserver and the browser on your computer communicates with the server using socket.io. The BBB then passes the control signals received from the network via serial link to an ATMega 2560 which directly interfaces with the Motor ESC’s, lights, servo etc

The DC voltage you are measuring on the tether is used to turn on an optocoupler on the ROV which turns on the rest of the ROV electronics,

You can find out a lot more about how the hardware and software work at https://github.com/OpenROV
All the code and schematics are available there.


#3

Hi @Brendan, thank you very much for your detailed explanation. It’s really interesting.

Here is my understanding:


For the original home plug unit:

  1. The power supply board collects 110 AC and step it down and rectify it into a 3.3V DC to supply power to the high-tech board. the J7 and J2 pins are the interface for RF communication.

  2. The 110 AC acts as a carrier wave which carries the RF signal generated by the high-tech board. I suppose the RF signal has a very different (higher) frequency than the 50Hz 110 AC.


For the modified home plug with top side adapter in our ROV:

  1. The power of the high tech board comes from the top side adapter’s mini USB port. roughly 5V DC.

  2. The 4.5V DC between the tether pins acts as a carrier wave (please verify my statement) carries the RF generated by the high-tech board to the ROV. The 4.5 V DC is also used to turn on the ROV.

speaking of modulation, is this some kind of frequency modulation (FM)? but usually, in FM, there is a low frequency carrier wave and a signal wave (usually with much higher frequency) to be modulated. In this case, does the 4.5V DC really behave like a carrier?

Please clarify my understanding!

Many thanks!


#4

Hi @sunjianxiao12.08:

Your description is close.

The communication protocol going over the tether (Homeplug AV) does not need any particular voltage to serve as a “carrier”. It will work perfectly fine over a pair of wires that has no DC voltage on it.

We take advantage of this to use DC voltage to signal whether the ROV should be turned on or not. If there is more than ~3V DC on the tether, the ROV turns on. Remove the DC voltage from the tether, and the ROV turns off.

-W


#5

Hi @sunjianxiao12.08
Hi @Walt_Holm

The homeplug generates it’s own carrier. It has a high pass filter to ignore any signals under about 1MHz, including mains voltages and DC. It’s modulation is based on FFT Orthogonal Frequency Division Multiplexing (OFDM). It’s part of the IEEE 1901 standard.

Thanks,
Brendan


#6

hi @Walt_Holm
hi @Brendan

Really thanks for sharing of your knowledge. It’s an eye opener for me!

Just to quote from wiki:
“OFDM is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier modulation method. A large number of closely spaced orthogonal sub-carrier signals are used to carry data on several parallel data streams or channels. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase-shift keying) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.”
“In OFDM, the subcarrier frequencies are chosen so that the subcarriers are orthogonal to each other, meaning that crosstalk between the subchannels is eliminated and intercarrier guard bands are not required. This greatly simplifies the design of both the transmitter and the receiver. In conventional FDM, a separate filter for each subchannel is required.”

now my only doubt is that for the original homeplug unit, the mains voltage is merely used to provide power to the homeplug board through a step down transformer and a AC to DC converter, it is not used as a carrier neither since the homeplug is able to generate sub-carriers.

Thanks
SJX