What EZB are you trying to connect to? There are 20 EZB types, so you'll have to specify. Since you're talking about a COM port, I'm guessing it's an Arduino.

Arduino COM ports are detected when the USB cable is connected to the computer's USB port. On your laptop, you would see a USB cable connected to it; on the other end would be an Arduino. What you can do is disconnect the part of the cable that's on your laptop and connect it to the USB port of the Rock Pi. The Rock Pi will now detect that COM port as the Arduino's s physical connection is directly connected to the Rock PIx.

A COM port is not a thing all computers have - they can be added, usually through USB. More about COM ports here: https://synthiam.com/Support/Advanced-Fundamentals/What-is-a-COM-port

Lastly, if you want to hardwire the onboard Rock PI COM port to the TX/RX of an EZB that supports it, I'm guessing it doesn't show up because you're missing drivers. There's probably some exclamation mark or question mark in the device manager about the port. Or, it isn't an OS-supported COM port. You can check Rock Pix's manual to see how to detect it in Windows. I know that my Rock PIx do detect the COM port in windows, so I'm guessing you're just missing a driver.

My EZB is EZB ver4/x2. What driver would be missing to prevent my Rock PiX from detecting the USB Com port? I note there are drivers provided in this link https://synthiam.com/Support/Hardware/Rock-Pi-X. But I see nothing helpful there. I will try to install a driver for the FT232RL USB to TTL converter I am using, that may be the problem.

Installing the drivers for the FT232RL USB to TTL converter did the trick. Rock PiX now shows COM3 and ARC can connect. Thanks for pointing me in the right direction.

Woohoo! Awesome