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I am creating an underwater ROV that will make use of a 40-50 meter CAT5 cable as a tether. I need to transmit commands/sensor information to and from a custom control board in the ROV while also sending camera data to the surface from a repurposed USB webcam. I want to use one twisted pair for the microcontroller on the ROV control board while using the second pair to directly send camera data. Ideally I don't want this data to be received and sent by the microcontroller in order to reduce MCU cost and overhead.

So far I have considered RS485 as well as single pair ethernet. The issue with RS485 is that it seems it won't support sending 720p video over more than say 30 meters (assuming a data rate of around 6.5 Mbps). Single pair ethernet is promising but would require two stages of bridge ICs for both the camera and microcontroller which would be costly. Any ideas on the most effective method to transmit these two data streams to and from the surface would be much appreciated.

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  • \$\begingroup\$ If 30m isn't enough range, how much range is needed? (defining your requirements will help get you a useful answer) \$\endgroup\$
    – Daniel Chisholm
    Commented Nov 24 at 21:57
  • \$\begingroup\$ A DSLAM modem to an ADSL modem can exceed your MHz-km requirements with digital video. Baseband video needs impedance matching 75 Ohms to 100 Ohm Cat5. Ethernet has 100 m limit. \$\endgroup\$
    – D.A.S.
    Commented Nov 24 at 22:09
  • \$\begingroup\$ aliexpress.com/item/… with mods to replace audio \$\endgroup\$
    – D.A.S.
    Commented Nov 24 at 22:36

2 Answers 2

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RS-485 also does not convert to USB, and USB can't pass through more than 5 meters in any case, and CAT5 has incorrect impedance for USB.

Send the video as analog over one pair. Use second pair for half-duplex RS-485 with any protocol you like (you likely mean UART, but RS-485 does not define a bit-level protocol).

It seems using something like cheap Raspberry Pi like board to plug in USB camera and communicate everything via 100 meters Ethernet could work.

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  • \$\begingroup\$ These are all good points, communication protocol was not the right phrase to use in my title. I know USB can't directly be sent across CAT5 but bridge ICs do exist that allow for sending usb data over ethernet such as the LAN9500. The issue with video as analog is the camera module I am using directly outputs serial data over USB. I do like the idea of a Raspberry Pi but am very space limited and ideally wanted to design a fully custom board to handle the communication and video transfer. \$\endgroup\$
    – Ethan Peck
    Commented Nov 24 at 22:19
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Cat5 cable has four pairs; you can use two pairs for Ethernet, or for POE (power over Ethernet) and network, and one pair for video of the analog type (PAL or NTSC if it has to be in color).

USB isn't a good interface for video if you have 40-50 feet of cable: USB2 allows 5 meters (about 18 feet) and USB3 is less.

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