This is not the site for P-series Digital Pianos… we answer questions if we can. Particularly, when it comes to buying a Synthesizer versus buying a Digital Piano. Some folks get confused between how they differ (and they very much do), so we offer help. We don’t mind answering if it is helpful…

The P515 comes with a Yamaha FC4A Pedal — just FYI
The FC3A or the dedicated LP-1 3-pedal unit are necessary for the advanced sustain pedal functions; (and are sold separately).
The FC4A does not do the advanced sustain pedal functions —You can quickly tell them apart: the FC4A has a TS connector, the FC3A has a TRS connector.

From the Owner’s Manual…page 18
Damper Resonance
When you select a Piano Voice, pressing the footswitch activates the Damper Resonance (VRM, page 27) to accurately recreate the resonance of an acoustic piano’s strings.
You can turn the VRM effect on/off and determine the depth of the damper resonance in the Piano Room display (page 25).

Half-pedal function
This function can be used when a separately sold FC3A foot pedal or LP-1 pedal unit (page 19) is connected and allows you to vary the sustain length depending on how far the pedal is pressed. The farther down you press the pedal, the more the sound sustains. For example, if you press the damper pedal and all notes you are playing sound a bit murky and loud with too much sustain, you can release the pedal half way or higher to decrease the sustain (murkiness).
You can specify the point to which you must depress the foot pedal connected to the [AUX PEDAL] jack or the damper pedal of the pedal unit (Half Pedal Point). Press the [FUNCTION] button repeatedly to call up the “System” menu display: “Pedal” > “Half Pedal Point” (page 99).

2. I want to record in a DAW the P515 CFX and Bosendorfer sounds post VRM processing. Do I get the best result by using the P515's audio interface ? Or is the result the same by connecting the P515 aux out to the MODX Aux In, and using the MODX Audio Interface?

You’d have to try it and see if there is any audible difference. If you use the the P515 as its own audio interface, this will eliminate the extra D-to-A and A-to-D steps that connecting analog to MODX would require. Is that extra step audible to the human ear… frankly I don’t know. It might be measurable but completely inaudible.

You have the gear… try it, let us know. They use the same driver, assuming the analog connection between the P515 and the A/D In on the MODX are quality cables, I doubt very much you would be able to tell a difference. What you gain by going through the MODX is a dedicated Audio In channel strip that would allow you to apply two Insertion Effects, Reverb, Variation Effects etc., and the ability to use the P515 as a modifier within the MODX’s Motion Control Engine.

Thank you for your response.

I purchased the FC3A (as noted in my post); it seems to be working the same as the FC4A provided with the P515.

(Edited: I tried the Half Pedal Point from -4 through +4; can't hear any difference, seems to be the same behavior, Thanks again for your time; I'll try to find a forum for the P515).

(Edited: I tried the Half Pedal Point from -4 through +4; can't hear any difference, seems to be the same behavior, Thanks again for your time; I'll try to find a forum for the P515).

You mean -2 thru +4… what you are listening for can be difficult to hear if you are not listening for it.

With the pedal 1/2 way down, you should detect some sustain. Hold the pedal there and play a few notes — let them ring…
With the pedal fully down, you should experience longer sustain times. Hold the pedal there and play a few notes — let them ring…
The difference should be the full on sustain pedal function should only occur when the pedal is fully engaged, and degrees less than full should be less sustain before it fades to nothing.

The pedal, when working properly, (I’m sure you know) sends continuous values 0-127 — these are used to dynamically control sustain values less than full (127) in sounds programmed to respond. A regular TS switch sends either 0 or 127, no grades of sustain in between… just off or on. Half Dampering is literally about shorter sustain values being available.

Test the FC3A pedal on your MODX, using the same routine… choose one of the MODX main Piano Performances… the MODX defaults to Sustain Mode = FC3A
[UTILITY] > “Settings” > “Sound” > Set the Sustain Mode = FC3A HALF (Dampering) ON

If it is the pedal this should tell you.

Thank you, sir; a righteous dude!

While I did know that the FC3A was a continuous pedal, seeing the '0-127' in print triggered the idea to test using an FC7, and yes, indeed, the P515 clearly responds with a sustain about midway through the FC7's 'toe down' movement. And there is really narrow band of midway'ish positions where the sustain 'rings' at a noticably reduced 'ring' time.

When reconnecting the FC3A at the same -2 half pedal point, I found I can find a similar point just before full depression. Very difficult to find. Validated the same behavior with the MODX (CFX Concert, FC3A-Half On).

Given that the design concept & construction is straightforward, I am going to assume it's per-spec, accept the behavior and treat it as a FC4A as I want a sustain pedal for both the MODX and the P515.

Thanks again for taking the time to help out with a not-a-synth question.

P.S. I am quite pleased with the P515: agree the CFX Grand voice is awesome with headphones, NWX keyboard action fits my touch very well and, damn, looks good! A few wish list items - I'll be seriously tempted to upgrade when Yamaha upgrades the P515 with a Grandtouch S keyboard (to reflect the Clavinova CLP 745) as well as a binaural Bosendorfer headphone voice. Nice-to-have would be onboard Registration capability, however, can use Camelot Pro to manage this.

2. I want to record in a DAW the P515 CFX and Bosendorfer sounds post VRM processing. Do I get the best result by using the P515's audio interface ? Or is the result the same by connecting the P515 aux out to the MODX Aux In, and using the MODX Audio Interface?
You’d have to try it and see if there is any audible difference. If you use the the P515 as its own audio interface, this will eliminate the extra D-to-A and A-to-D steps that connecting analog to MODX would require. Is that extra step audible to the human ear… frankly I don’t know. It might be measurable but completely inaudible.

You have the gear… try it, let us know. They use the same driver, assuming the analog connection between the P515 and the A/D In on the MODX are quality cables, I doubt very much you would be able to tell a difference. What you gain by going through the MODX is a dedicated Audio In channel strip that would allow you to apply two Insertion Effects, Reverb, Variation Effects etc., and the ability to use the P515 as a modifier within the MODX’s Motion Control Engine.

Just spotted your response to my 2nd question. Thank you. Good point - may be measurable but not audible enough to make a difference, e.g. on a demo/backing track. And going through the MODX audio interface gives me interesting production possibilities.