With a few tweaks I finally got the Arduino Mega to correctly talk to the BeagleBone. I gave up trying to get it to work over USB, and wired Serial 3 from the MEGA to UART4 on the BeagleBone. I can now get the ultrasonic distance measurements. This now allows me to complete a feedback loop between getting measurements and physically moving the robot. I took some video. I’ll get that processed eventually.