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RS485 over Fathom-S?

(Richard) #1

Hi All
Just when I think I’m getting a grip on serial communications, some new puzzle shows up. Here’s what I’m thinking about today: I’m considering adding a Fathom-S setup to my BR2 which already has a Fathom-X setup. Why, you ask? I’d like to have the analog camera on board as a backup/rear-view//low-light option, and I also need RS485 communications. Before I thought of the analog camera idea, I was just planning to use one of the three spare wire pairs in the tether for the RS485, but when I looked at the Fathom-S specs it appears that it can do RS422, which is pretty similar to 485, so maybe it might be an easy solution for both. Looking in more detail at the S boards, it appears that they provide 5V TTL serial, which is a different animal than the RS standards. Here are the issues as I see them:

  • Is there a better way to get the analog camera’s signal up the tether that doesn’t involve the Fathom-S boards? Like maybe I could do that with a single pair, RS485 over another single pair, and still have myself a spare pair versus using three pairs to do it the Fathom-S way?

  • Can I in fact do RS485 with a Fathom-S setup, and if so, what would that wiring look like? TTL-to-485 converters at both ends?

(Rusty) #2

Hi Richard,

What are you actually connecting to the RS485? If you’re trying to add an RS485 device, like a sonar or something, you can just connect a twisted pair directly to that device - you don’t need the Fathom-S. You could still use a Fathom-S for the analog video and just leave the rest of the wires on that board disconnected.

The Fathom-S translates standard TTL serial, which you find on devices like most microcontrollers, Arduino, etc., to RS422, which is basically the same thing but travels much longer distances. It really depends what you’re trying to do.

For your questions:

  • We are planning to support multiple digital camera feeds in the relatively near future. For now, you can do analog video over a single twisted pair with the Fathom-S and RS485 on a different pair.

  • If you want to do RS485, you should completely bypass the Fathom-S and connect directly on both ends to your device and your RS485 to USB converter.


(Richard) #3

Heya Rusty, thanks as always for the speedy reply. I’m communicating with sonar and a grabber over RS485, so I’ll keep that separate on its own twisted pair. How can I rig up analog video over a single twisted pair with the Fathom-S? Or put another way, which pair on the S boards carries the video signal?

(Rusty) #4


Only the first (blue) twisted pair is used for video. You can leave the rest disconnected as long as you have the jumper positions for “always on” power.


(Richard) #5

Perfect, I will do exactly that!