The PK7 is sort of like placing a piano keyboard at your feet. Although the PK7 has a MIDI Merge function - the primary function of the device is to send MIDI --out-- from the pedal board TO a keyboard of some sort in order for your feet to play notes.

What the PK7 receives is sort of irrelevant if you want to use it to play strings and cello. This is because, in order to accomplish this task, you're really only interested in hooking up the MIDI OUT of the PK7 to the MIDI IN of MODX.

... maybe you said "receive" and meant to say "send". That would make more sense to me.

Make sure the MIDI mode of MODX is MIDI and not USB. There's an icon at the top of the touchscreen which will either look like a circle with dots (MIDI) or a USB logo. Touch that to change the setting if you need to.

You're going to want to set the MIDI channel of the PK7 up. By default it is set to MIDI channel 2. Apparently you have to set this up every time you power it on - it seems like it is reset to CH 2 when you power it off.

Also, the note range output from the pedals cannot be adjusted. This is kind of limiting because you cannot, for example, assign the note range below the range of your 88 piano keys and then use note range as a way to differentiate notes from the bass pedal.

The bass pedal outputs notes from 36-55. This simply does not allow for non-overlapping notes since you cannot shift a full 88 keys above 55. (55+88 = 143 which is above 127).

If you want the full 88 without the bass pedal you have the following choices:

1) Set the bass pedal to a channel - say Ch 8 (you do this by pressing the hold button down at the same time as the 8th pedal from the left during power-on you cannot have anything plugged into the "Hold OUT" port). Then place your Strings+Cello in Part 8. You'll need to have MODX set in Multi-channel MIDI mode. You can only have one PART controlled by the bass pedal while in Multi-Channel MIDI mode (without adding additional hardware).

2) Modify your piano part to stop responding at velocity 119 (set velocity range to 1-119). You can use velocity scale to ensure the hardest you hit the keyboard will only register as a 119. The bass pedal outputs only a single velocity - 120. So you can have any of the PARTs you want to respond to the bass pedal set to only respond to velocity of 120 (velocity range = 120-120). This way - you could setup your keyboard in single-channel MIDI mode and set the transmit/receive channel to "2" which would match the pedal default.

Note: that you could have more than just a piano sound under direct keyboard control. Any PARTs you add - you would adjust the velocity the same way (1-119). These would ignore the bass pedals.

There's other possibilities which start getting harder to deal with in programming and performance.