MODX OS v2.00 added MIDI Data Reception with USB TO DEVICE (check the Supplementary Manual).

On my Montage I have an 88-key hammer action MIDI controller plugged into USB TO DEVICE and it's working fine for me.

Correct, you can make the connection from the P125 “TO HOST” port directly to the MODX’s “To Device” port using a standard A-B type USB cable; __ this will allow you to play the P125 keybed and trigger the MODX synthesizer — the P125 keys virtually replace the keys of the MODX6.

You will still probably want direct access to the MODX front panel for things like Pitch Bend, Modulation, Assign Knobs, Super Knob, etc., etc., etc. You’ll want to look into a two tier stand or situation where you can easily access the MODX front panel.

Of course, you could trade in the P125 and move up to the MODX8 which gives you a P125 equivalent keybed, and the MODX engine in one convenient lightweight package. (( I have to mention that, just in case you were unaware the MODX came in an 88-key weighted action version )) just FYI

Thank you for the info about connecting the two.

I have read the manual and it states that you can only use one midi connection at a time on Page 54 "Either of the built-in interfaces, the MIDI terminals, or the [USB TO HOST] terminal, can be used for MIDI data transmission/reception; however, they cannot be used at the same time." Is this still true after the update? I was thinking of connecting my Korg Minilogue XD by MIDI also.

Bad Mister. Yes I know they have an 88 key version, but it's $600 more.

The MIDI I/O selection is still one or the other; the USB TO DEVICE support is its own independent thing (you can still control the synth via it even when the MIDI/IO mode is set to MIDI).

Do note that another device connected via USB TO DEVICE does not behave the same as if you connected it to the MIDI IN port; it has its own special behavior. That may or may not be an issue depending upon what you wish to do.

Note that the "one or the other" business (5-pin DIN MIDI -OR- USB MIDI) is a little misleading (I stress little - because it's not entirely wrong) because you can actually simultaneously access MODX by USB (using USB PORT-1) while also accessing 5-pin DIN connected instruments (using USB PORT-3). And logic on your PC/Tablet can "connect" the 5-pin DIN side to the USB-MIDI side by way of message forwarding/routing. They're not directly internally connected - but that is not a limiting factor as both are active and available at the same time when MODX is configured for USB-MIDI.

What is true is both cannot be used for transmission/reception DIRECTLY to-and-from the MODX tone generator and MIDI engine. However, both are active at the same time and can be "connected" by way of an external device assuming using USB-MIDI.