I’m using Blue Robotics Fathom Tether cables for Ethernet communication with my underwater vehicle. I originally used a 30 m tether, and everything worked perfectly. Later, I switched to a 90 m Fathom tether, which also worked reliably at first.
Recently, I’ve started seeing intermittent packet loss on both cables:
Sometimes all packets go through
Sometimes only half
Sometimes the packet loss increases suddenly (e.g., from 4 packets → 2 packets → 3 packets → 2 packets)
The link never fully disconnects, but packets are being dropped inconsistently.
Has anyone experienced intermittent Ethernet packet loss over long lengths of Fathom Tether cable (30–90 m)? Could this be due to cable attenuation, aging, or connector degradation, even if the cable initially worked well? What troubleshooting steps or solutions would you recommend?
Given it’s a notable change from previous behaviour, I’d expect one of:
You have changed what you’re sending through it, and it is now above its capacity, so any given packet has a chance of being dropped in favour of other ones
There are one or more breaks in the tether wires, which are being held in place by their insulation+sheathing, so they cause intermittent dropouts depending on how the tether is bent/twisted/pulled/moved
There is corrosion on one or more of your connector pins, and/or on one of your wires (e.g. if the cable sheath has been cut/nicked and allowed water ingress to the cable)
Assuming you’re using Fathom-X* boards on either end, and BlueOS in your vehicle, you could try installing the Tether Diagnostics Extension and seeing what kind of connectivity you have?
*Note that the HomePlug signal protocol used by Fathom-X boards has a side-effect of travelling surprisingly well along nearby conductors via parasitic capacitance, so with a 4-pair tether even if your primary pair is completely severed in the middle, or you’ve plugged each end into a different pair, the signal may still through (albeit generally with reduced quality).
Other than that your main options are network speed tests, ping checks, and resistance measurements of the conductors.
Thank you for the detailed explanation — this helps a lot.
To add more context, yes, we have increased the amount of data being sent recently, so it’s possible that the higher data load is now pushing the cable closer to its limits.
Also, just to clarify, we are not using a Fathom-X board. The tether is connected directly from the Raspberry Pi to a networking switch, and from the switch to the GCS (laptop). So the path is:
Hi @Harshada -
I’m surprised you get a connection at all over 90m of Fathom tether via Ethernet! Because it uses stranded wire for flexibility, it does not meet the specifications for standard CAT5 cable- I’ve never seen a connection longer than about 60-70m function.
Shortening your tether, using a different type that uses solid wires, , or installing a tether adapter like the fathom-x should resolve your communications issue!
Can repeated cable bending, flexing during deployment, or changes in ambient/water temperature cause internal cable issues that lead to intermittent Ethernet data loss, even when basic tests pass?
Yes, especially if you’re bending the cable more sharply than its specified minimum working bend diameter (which for the standard Fathom Tether is 75mm), in which case there may be data loss during those bent periods, and/or the wire strands may snap resulting in the kind of intermittent dropouts I described before:
Thanks for the clarification on the Fathom tether behavior.
I’m planning to switch my communication link from Ethernet Fathomcto RS485 (using RS485 modules on both the ROV and GCS). Will this require major changes to my existing hardware and firmware/software? Also, will switching to RS485 significantly reduce data rate or introduce noticeable latency to the GCS? Since RS485 is typically half-duplex, do I need separate modules for transmission and reception on each side, or can this be handled with a single module per end?
Hi @Harshada -
How do you intend to carry the video signal? RS-485 is not going to have the data rate required to move more than telemetry…
If you’re having issues with reliable communications with your vehicle, using the Fathom-X tether modules will greatly improve the situation - as Fathom tether is not technically ethernet cable, and so does not work well at greater distances. Fathom tether uses stranded wire, while the Cat5/Cat6 specification calls for solid core. While things may work coiled up tightly on the bench, packet loss and othe rissues abound when uncoiled!
Understood. RS-485 is not intended for video transmission and is only being considered for low-bandwidth telemetry. At present, we are not receiving any video at the GCS at all, and the only data coming through is in the kbps range.
Does the Fathom-X board (or the LX200V20 module on it) support RS-485 communication natively? Or does it work only with other protocols (e.g., Ethernet-style via HomePlug/1901)?
Hi @Harshada -
The Fathom-X only provides ethernet connectivity, using a single twisted pair, at great distances. Using it with your existing tether should provide both video and telemetry, once installed on both ends and with the link light illuminated.
You’re not getting a reliable ethernet link with sufficient bandwidth while currently using the Fathom-X tether for standard ethernet signals, because the tether is not constructed to carry these standard signals over the lengths you’re using…
RS485 would only make sense as an alternative if you had another means to send the video signal - analog perhaps? Additional configuration of BlueOS to output serial telemetry would also be necessary… definitely easiest to use the Fathom-X tether module!
Okay, Whether using a Fathom tether along with Fathom-X boards on both the vehicle and topside will provide a reliable Ethernet link over the full tether length. Specifically, will this setup ensure stable data transmission without packet loss, random disconnections, interruptions, or noticeable latency, when carrying both video and telemetry together?