A lot of laptop computers and many USB-to-serial adapters actually just
swing +5V and 0V for the two states. It's a real problem for gear that
is strict about the standard, and may be why you have problems powering
that device there. Maybe the Digi box will swing to -5V for low, and
that gives you 10V to power it? I've not seen any adapters that are
only doing 5V that actually go to -5V on the low side, so this may mean
that box wouldn't work there anyway.

1. The newer box can be powered by external power and also Power over Ethernet (PoE).  The older box does that, and also can be powered from RS-232C RI ("Ring Indicate").  However, I'm only powering both boxes with external power (which can be anything from 9-30vDC, and I'm supplying 13.8vDC).
2. Both boxes can support both RS-232C, RS-422, and RS-485 via switch settings, but only the older box has switch settings for each serial port.
3. The newer box works with all my radios with DTR and RTS high.  The older box works the same, except that for the CT-29A cable, it requires RTS=high, DTR=low.  If the older box sends DTR high to the CT-29A cable, the cable responds in such a way in such a way that the Digi box locks up (which means all serial ports on the box go down).  The Yaesu CT-62 serial cable from the FT-897D gets its power from the radio, and the OPC-1529R cables from the Icom D-Star radios are just TX/RX/GND wires (DTR/RTS are unconnected), so those radios work just fine with either Digi box.