In some cases, you might want to connect a third party 232 or 485 device such as sonar, thickness gauge, flooded member probe ect through the ROV.
Such device might come with a software that requires a connection via a com port.
Traditionally, you would need an RS-232 converter subsea and an RS-232 converter on the surface and a line driver, fiber mux etc in between.
You can push these devices using RS485 which require extra wires in your umbilical.
There is another way which is very simple and cost effective.
The first thing you will need is a network switch in the ROV. I sell this on my website for those who want an already made solution: http://www.deltarov.com/new/product-category/rov-accessories/communication/
You then connect the switch to the Raspberry pi and Fathom-X. Although I recommend you replace the fathom-X and cross over to fiber. (Also available on my website)
You then buy a USR-TCP232-T2 from ebay, aliexpress or any other source you prefer.
This device is under 10USD
You then buy a 232-ttl converter.
This is under 1USD
Once you have everything connected, you can download the software and manual.
You can find this online or download it from my website: http://www.deltarov.com/new/download/
I made a quick video to show the results. Sorry if its not very professional, I just wanted to share the results.