What is the idea behind that? It can be interpreted as: 
1. only show ports that are in use
2. show ports that are in use in addition to all other ports
3. display an indicator for ports that are in use

I suspect the latter two, so I'd rephrase to: "… RECOMMENDED that the user can select from all serial ports available on the machine, and be shown an indicator for serial ports that are currently busy, …"