What you're really talking about here is the need for a communications driver to interface between the DAW and the outside world. that's exactly what OMS and/or FreeMIDI do - or in the case of really old stuff, Apple MIDI Manager.
According to MOTU:
MOTU USB Interfaces and OMS
You can use you new Motu USB MIDI interface with OMS if you need to. After the latest version of OMS 2.3.8 is installed, you should install FreeMIDI 1.4 (or later). If you are on a G4, iBook, or DVD iMac you will need FreeMIDI 1.43. The FreeMIDI installer will place a MOTU OMS driver in the OMS folder, which can be found in the System folder. After restarting the computer, you can now run your OMS setup and detect you MOTU USB interface by hitting the search button when OMS prompts you to choose printer or modem port.The bottom line is that there is a MOTU OMS driver extension and also an Opcode OMS USB extension to "bridge the gap" between the old serial port world and the later USB world. Some combination of the above stuff will almost always work.
A couple of notes:
* The "gold standard" of PCI serial port replacers is the Keyspan SX PRO which came in both two and four-port versions. They're scarce as hen's teeth.
* Acquiring such a device will cause you to need to switch to a serial-based MIDI interface like an Opcode Studio xx instead your existing MOTU interfaces.
* Although
theoretically you can use both a serial card and interface plus a USB interface simultaneously, I've yet to meet anyone who does. I can only assume that's because of (A) unwanted interactions between them or (B) maybe I just don't get out enough…
and finally…
I can see that all of this appears to be driven by your apparent lust for the Akai 612. I submit that being involved with either old Akai or old Roland/Ederol stuff today is a shortcut to hair-pulling insanity as you try to drag them kicking and screaming into a future they were never intended to see.
I'm just sayin'…
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