Questions about the Electronic Architecture of the ROV2

General Discussion
1

Hi,

I have some questions about the electronic Architecture of the ROV2.

I was wondering why there are two Fathom-X and a RS485 link between the computer and the Raspberry3 ?
Could an only RJ-45 cable be used instead of that part, even if there is power loss due to the lenght of the cable ?

Best regards,

Pierre

2

Hi Pierre,

Thanks for the question. First, let me clarify, the only connection between the topside and the ROV is through the Fathom-X boards (and just two wires). There is no RS-485 unless you add it to support something else. The tether does have extra twisted pairs for that.

You can absolutely replace the Fathom-X connection with a standard Cat5 Ethernet cable. There are a few primary reasons that we use the Fathom-X boards:

  • Uses only 2 wires instead of 8 wires, making for a simpler tether.
  • Can handle much longer distances than regular Ethernet, which is limited to about 90 meters.

I hope that answers your question.

-Rusty

3

Hi Rusty,

Thank you a lot for your answer. It is very clear now but I still do not understand how the Fathom-X allow to use a long distance connection (over than 90m Ethernet). Does it boost the signal power ?

Moreover, I have another question.
I find difficulties to calibrate one of the ROV2 I built.
The accelerometer works good but I can’t calibrate the compass. Is that an usual issue ? How could I fix it ? Does it mean that there is a problem with the Pixhawk ?

Thank you for your answer.
Have a nice day.

Best regards,

Pierre

4

Please follow these tips about calibrating the compass, and let me know if it works.

5

Hi Pierre,

The Fathom-X board uses a “HomePlug” communication module, which converts the signal to a different type of signal modulation and sends it over two wires. It is converted back to Ethernet on the other end. This signal type is very robust so it is able to travel over longer distances reliably.

-Rusty

6

Hi Jacob,

You were totally right.
These tips were really helpful.
The Rov2 is working now.

Thank you a lot.

Best regards,

Pierre

1 Like
7

Hi Rusty,

Thank you for your answer and for your time !

If I have other questions, I will not hesitate to contact you again.

Best regards,

Pierre

8

I’ll admit, I have used ethernet, but don’t understand the physics behind the limitations of cat5 cable. If you use a Cat5 cable for ethernet, your longest guaranteed transmission length is 100 m according to spec. As far as I can tell, the fathom tether is similar to a Cat5 cable, but can be used with the Fathom-X interface for lengths as long as 300 m.

I am trying to understand how you managed to triple the transmission distance while maintaining wire gauge. Is this due to the engineering of the fathom cable itself or is this also possible with standard Cat5? I know crosstalk usually causes interference between the twisted pairs which causes signal degradation over long distances. Because homeplug only uses one twisted pair, I assume this crosstalk is eliminated. Is the increased transmission distance due to this simple change? If not then what is the reason for the increased transmission distance?

9

It is not the cable that allows 300m, but the signal modulation technology. We use a technology called homeplug that allows ethernet communications over a single twisted pair. Homeplug also allows network comms over cable lengths greater than 100m.