My senior capstone engineering team is currently making an ROV that uses an Arduino Mega for interpreting data sent from a computer (Windows) via USB, and using that data to control the ROV thrusters (ESCs with brushless motors). In addition to this, we also have a second computer that is on board the ROV (Beagleboard-XM, Ubuntu 14.04 LTS) used for fully autonomous control for graduate research.
Since we are supporting the graduate research effort, we must include the Beagleboard-XM on the ROV so more advanced control algorithms can be computed. So, the goal is to handshake between the windows machine and the Begaleboard-XM and at the same time send commands to ESCs to move thrusters. We can use the standard USB connection to the computer for the first serial port, but for the second, can we splice an existing USB cable for the Beagleboard-XM and just wire it into the TX and RX pins for the second serial port? Is it just that easy? If so, what would the port location be on the Beagleboard-XM side? /dev/ACM0 still?
Would one of these products work to do what we are looking at? Is the full FTDI needed if we will not be programming the Arduino through this connection? We only want to use if for communication to and from the on board computer.
Yes, you most certainly can use an FTDI cable (or one of the many other chipsets which may be more reliable) to "talk" to the second UART on the MEGA250.
It might however, be more practical to simply issue all communications through the on-board computer by default/ design.