Fathom-S Documentation


The Fathom-S Tether Interface Boards provide a video and communication interface between your ROV and topside. Differential analog video and RS-422 communication provide a reliable, low-latency link and the flexibility to be used in many different applications.

The Fathom-S hardware is designed to be used with tether cables that have four twisted pairs, preferably similar to Cat5 cable. The design is based on the tether cable available from Blue Robotics, but will also work with standard Cat5 network cable.

The Fathom-S interface includes DroneCode Mini compatible connectors for easy use with the Pixhawk and other autopilot controllers and the ArduSub Control Software.


  • Low Latency Analog Video through NTSC or PAL (1000+ ft, 300+ m)
  • Adjustable Video Gain for different tether lengths
  • Full-duplex RS-422 serial connection with TTL and USB connections (up to 250 Kbps, 4000 ft, 1200 m) (3.3V logic, 5V tolerant)
  • Power switch that uses a 3-50V input signal to turn on interface (and optionally the autopilot and/or a high power switch/relay)
  • Onboard 5V and 12V (camera) regulated supplies
  • Jumpers to enable/disable configuration options
    • Power-on through tether / power always on when batteries connected
    • Power cycle on USB RTS signal for Arduino programming
  • DroneCode Mini Compatible JST-GH and DF13 connectors

Quick Start

  1. Connect tether cable to the terminal block, matching colors with the labels
  2. Connect the ROV side serial port to the ROV controller (Arduino, Pixhawk, etc.)
  3. Connect an analog camera to the ROV side camera input
  4. Connect the topside board to an analog (NTSC/PAL) display and a computer USB port to view video and communicate through serial


Function Diagrams

Fathom-S ROV Board

Fathom-S Topside Board

Specification Table

Item Condition Value
Supply Voltage 7-20 volts
TTL Serial Voltage (ROV Board) 3.3 volts (5 volt tolerant)
TTL Serial Voltage (Topside Board) 5 volts
Camera Supply Voltage Vin > 12V 12 volts
  Vin < 12V Vbat
Max 5V Load Current Vin @ 12V 250 mA
  Vin @ 16V 150 mA
Max 12V Camera Current Vin @ 12V 600 mA
  Vin @ 16V 250 mA
Switched Power Voltage (SW Header)   Vbat
Switched Power Current (SW Header)   2A
Max Serial Baudrate   250 Kbps
Max Tether Length (RS422)   1200 m
Max Tether Length (Video)   300 m
Operating Temperature   -20 to +85°C
Storage Temperature   -40 to +85°C
DF13 UART Pinout   DroneCode Mini Standard (crossover RX/TX)
JST-GH UART Pinout   DroneCode Mini Standard (crossover RX/TX)
USB Connector Type   USB Mini B Female
Dimensions (ROV Board)   64 x 46 mm
Screw Hole Spacing (ROV Board)   57 x 40 mm
Dimensions (Topside Board)   48 x 45 mm
Screw Hole Spacing (Topside Board)   42 x 40 mm
Screw Hole Diameter   3.3 mm


The EagleCAD files for the schematic and board are available on our GitHub page.


2D Drawings

Fathom-S ROV Board

Fathom-S Topside Board

3D Model

All 3D models are provided in zip archives containing the follow file types:

  • SolidWorks Part (.sldprt)
  • IGES (.igs)
  • STEP (.step)
  • STL (.stl)
Fathom-S Topside Board FATHOM-S-ASM-TOPSIDE-R1.zip


To Pixhawk

To connect to a Pixhawk autopilot, use the DF13 cable included with the Pixhawk to connect the DF13 UART connector on the Fathom-S ROV board to the Telem1 connector on the Pixhawk. Note that you must power the Pixhawk with the Pixhawk power module or another method as the Fathom-S board cannot supply sufficient power.

Fathom-S and Pixhawk

To Arduino

To connect to an Arduino, using the header pins on the Fathom-S board and connect the following:

  • Fathom-S RX to Arduino RX
  • Fathom-S TX to Arduino TX
  • Fathom-S 5V to Arduino 5V
  • Fathom-S GND to Arduino GND

Fathom-S and Arduino

Video Gain Adjustment

The video gain potentiometer allows the video receiver gain to be adjusted for different tether lengths. This makes a relatively minor difference in video quality but is noticeable over widely different lengths.

Fathom-S Gain Potentiometer

The potentiometer can be adjusted for gain suitable for 10m length to 700m length.


Arduino Programming

The Fathom-S board includes features to allow an Arduino microcontroller to be programmed through a tether. To enable that feature:

  1. Install the RTS RESET JMPR on the topside board

  2. Make sure the ROV board has a jumper installed for TETHER PWR-SW

  3. Connect the Arduino serial port to the tether board via the GND, +5V, RX, and TX pins. Make sure the Arduino is only powered via the tether board. This is required so that it will reset when the tether board resets.

  4. Select the appropriate serial port in the Arduino IDE and program like normal!

Spare Twisted Pair

In some applications, an addition twisted pair may be needed from the tether for other devices. In this case, you can disconnect Pair 4 from the tether boards and use it for other applications. This will disable the power-on feature so ALWAYS ON jumper must be installed.

Connecting Relay or Switch

The Fathom-S board includes logic to turn itself on through the tether connection. This can also be used to turn on an entire ROV by powering a relay or MOSFET switch. The SW pins provide a switched battery connection for this purpose.