embedded linux- R/W usb port question

I am new to Linux. I need to test embedded linux devices that have a USB and network ports. I have access to a terminal window. To test the ports I would like to connect a loopback cable to the usb and network and simply send a few characters and see if they can be read back. How can I do this in Linux. I have a C background. I am looking for manual commands to send in the terminal. I will create a script later. here are the usb devices:

root@TSSC:/$
lsusb
Bus 001 Device 001: ID 1d6b:0002
Bus 002 Device 001: ID 1d6b:0002
Bus 001 Device 002: ID 0424:7500

Jon K.

Port Testing and Loopback Cables

Port Testing and Loopback Cables - Other-Components - CPU-Components