Good day,
It seems, on the legacy ‘Edgeport/421’ devices, the ability to power the device directly from the USB cable may not be present by default. I found this out the hard way and, not wanting to search for a negative-center pin AC adapter, I dug into the unit itself with an eye towards modding it.
I’m happy to say I was successful. Note the attached photo. Running a 24AWG jumper from J10-VB to J9-VCC brought the unit to life when the USB cable was connected. A small piece of Kapton tape and a warning label can prevent plugging in of the now-unnecessary AC adapter.
Not bad for ten minutes of probing and a bit of soldering, hmmm? 
Happy tweaking.
