Check your general approach by loading the Performance factory preset "Init Normal (AWM2)". Touch PART 1, Edit, then "Mod/Control" -> "Control Assign", turn on "Auto Select", then turn the pitch wheel and move the foot controller(s). See if this works here.

I tried forcing this condition by turning off receive switch for FC1/2 and PB - that didn't do it. I tried various things to try to get this to happen and could not.

All that I know is that the [+] will go away if you've used up all 16 destinations. "Mod/Control" -> "Control Assign", press "Display Filter", choose "All", and page through destinations to ensure you're not at/near that water-line of 16 destinations. Each PART gets its own 16.

When I choose Display Filter under Edit > Part1 > Common > Control Assign

Try actually going to PART 1's Control Set (you are likely viewing the upper Common/Audio Control Sets... and not Part 1... here’s how to know:

From the HOME screen
Press the [PART SELECT_MUTE/SOLO] button on the far right front panel
Select "PART 1"
Press [EDIT]
Touch "Mod/Control" > "Control Assign"

Now you will be viewing the PART 1 Control Assignment...

The top line of the screen (which always shows exactly 'where' you are) will read "Edit - Part 1 - Common"
As you have noticed the available Controllers are different for the "Edit - Common/Audio" level of the architecture and the "Edit - Part X - Common" level of the architecture.

On the upper "Common/Audio" level is where you can affect ALL Parts - what is available is the Super Knob's 8 (Common) Assign Knobs 1-8, Motion Sequence Lane 1-4, Envelope Follower 1-16, AD, Master. The upper level is over ALL the Parts together, the entire Performance... here you are assigning control over things like the System Effects (Rev, Var), the Master FX, the Master EQ, the A/D Input, the Insertion Effects assigned to the A/D Input. Example, if you assigned a control to change “Reverb Time” (as the Destination parameter) you can understand that all Parts would be affected. Each Part has a Rev Send amount (that is a Part parameter) to the Reverb... but all Parts would share the result of a change in the Rev Time.

On the PART level is where your ‘usual suspects’ (standard Controllers are found): PB, MW, Aftertouch, Foot Control 1/2, Foot Switch, Ribbon, Breath Controller, (Part) Assign Knobs 1-8, Assign Switch 1/2, Motion Sequence Lane 1-4, Envelope Follower 1-16, AD, Master. The Part level is for just the selected Part. The parameter Destinations are per Part — and this is the real power of this engine — each Part has its own 16 Source/Destination Control Sets. This means you can define using the Curve, Polarity, Ratio, and ‘shaper’ Parameter how the control is applied.

Thank you both for your responses. Jason, it was the fact that the performance I was using had 16 assignments already made. Once I deleted one of the assignments the plus sign came back and allowed me to assign the controller I wanted.

What I failed to notice was that I wasn't seeing a plus sign for the assigned controllers that were coming up, because they had assignments already. I mistakenly thought that I wasn't allowed, for some reason, to use the specific controllers. Now I get it.

Thanks again.

No problem. That's the only thing that made sense to me. I had considered you were in the Common/Audio area - but that area doesn't show the controllers you were saying you were seeing -- so that was a no-go theory. I do think the interface could communicate a little better in this situation. Maybe a red "X" in the Destination area with a message "Destinations Full!" or some such thing.

The fact that you have ran into this limitation - and are the 2nd published user to do so - may contradict the notion that 16 is enough. My description of this spec as a "limitation" was met with much push-back.

You will also notice that when you run out of destinations - the [CONTROL ASSIGN] button will stop illuminating when you select parameters that otherwise would be able to be assigned to controllers. It might be something to consider for the future for this [CONTROL ASSIGN] button to have a 2 color LED - GREEN and RED. Combine they make amber (like the current). RED could mean you're full. I've suggested maybe a blinking state for this LED if you were out of destinations. Others do not like blinking at all. And, in this case, blinking would not be an error condition - just a "you're full" reminder - so not optimal. A red LED would be better - but I think the LED is currently just one color. Sorry about the tangent which should probably be transferred over to ideascale (from 2nd sentence of this paragraph - and the red "X" deal in the 1st).

On the PART level is where your ‘usual suspects’ (standard Controllers are found): PB, MW, Aftertouch, Foot Control 1/2, Foot Switch, Ribbon, Breath Controller, (Part) Assign Knobs 1-8, Assign Switch 1/2, Motion Sequence Lane 1-4, Envelope Follower 1-16, AD, Master. The Part level is for just the currently selected Part. The parameter Destinations are per Part — and this is the real power of this engine — each Part has its own 16 Source/Destination Control Sets. This means you can define using the Curve, Polarity, Ratio, and ‘shaper’ Parameter how the control is applied.

The one other “published user” actually had a MONTAGE, as I recall it, Jason.
There are 16 Destinations per the COMMON/Audio level of the architecture, and 16 Destinations per each Part at the Part level of the architecture. The Destinations are numbered 1-16. As you add one the Destination number increments to keep count!

If you have updated MONTAGE firmware (meaning 2.50) your “red x” (sic) is in the form of a warning that you have no Controller Destinations left when/if you attempt to exceed the limit. This was added in direct response to the one other “published user” who did not see the numbered Destinations in the Control Matrix. If I recall, they started building a MONTAGE Performance with an FM-X Template that had pre-assigned the 16 Source/Destinations as part of a tutorial... the tutorial that went with it explained all the Control Knob assignments were pre-made.

It’s certainly not that they programmed their own Control Sets and ran out. Not that this is impossible —it is just that this is NOT what happened. Quite the contrary, the template had pre-made the 16 Control Set assignments, so the user could not add any. The template was designed to give a function to all the Assign Knobs (for those challenged at making their own and so you’d have an FM-X Knob Setup ready to go). It was (expressly) not a template for the user to add additional Control Sets. But one to show hands-on control over the FM-X engine!

But in response to that issue, the warning message was implemented for those occasions when you do actually reach 16 Destinations and attempt to add another Control Set.

If this is not yet in the firmware of the MODX, it soon will be (as the MODX is typically one firmware innovation back from its big brother, the MONTAGE)...

I have not run into it in actual practice when building a sound from scratch. But it was deemed there should be a warning of some kind should you arrive there...
You don’t need a “red x” every time you exceed a specification, that’s not cool... the psychological damage caused by red x’s is under study... we’ve had suggestions of a loud annoying game show buzzer that sounds.

I remember an early sample based keyboard that released a string library set. Each solo string meticulously sampled... And they were troubled by the number of returns and complaints because on the violin sound there were no notes below G2 (the G below middle C)... the programmer lamented that keyboard players needed to get out more... so to make a point, he created a loud annoying sample that he mapped to the keys F#2 and below, when you touched one of those keys you heard at full volume a nasally voice scream “Out of Range, Out of Range, no violin down here, Out of Range!” ... it never got released, but funny as all get out on the programmer’s side of the table. Moral: some warning are really not necessary... but in the case of the MONTAGE, the warning message was added!