1) It doesn't seem to be recognized when plugged into the "Sustain" port (maybe the s80 is so old that the sustain port won't even recognize a pedal that isn't just on/off?)

You are correct. The S80 came out in the Summer of 1999... back then, in the 20th Century, the Half-Dampering function had yet to appear on the Yamaha Synthesizers. Please see your S80 Owner’s Manual (page 18) for information on recommended pedals (FC4 or FC5) both are TS connectors (Tip-Sleeve). The FC3A (TRS connector, Tip-Ring-Sleeve)) was introduced to the S-series in 2005 in the S90 ES.

The FC3A pedal cannot be (properly) used with your S80.

2) When hooked up to the "Foot Controller" port, I can assign the FC to MIDI CC64 and get a full range of sustain midi data, BUT it has the reverse polarity problem where sound is sustaining when the pedal is up, and when the pedal is down the sustain is off. (I also have an FC7 pedal plugged into the "Foot Volume" port, and that is working perfectly fine.)

How it behaves when plugged into the wrong jack, while curious, does not/will not make it work as the proper Sustain Pedal.

Sustain is cc64. The way it works in the S80 is as follows
All values 0-63 = sustain Off
All values 64-127 = sustain On
But it is the wrong connector (TRS) that will prevent your synth from recognizing the FC3A as a Sustain (on/off) pedal.

You need a Sustain pedal, like a Yamaha FC4 or FC5 (Or an FC4A) plugged into the dedicated Sustain pedal jack... These all have a TS plug to connect to the Sustain port.
The TRS connector is not recognized by your synth’s Sustain jack
— the Foot Controller ports are designed for Yamaha’s FC7 sweep pedal (TRS jack) which is why when you plug an FC3A into them they recognize minimum 0 through maximum 127. But no one really needs a spring loaded pedal that works like this... it’s just a curiosity.

Half-Dampering requires not only the proper pedal but a keyboard that can interpret the value (continuously) 0 through 127 for Sustain. This ‘magic’ is accomplished by combining the right pedal with a compatible product.
You should use the standard TS Foot pedal, as recommended.
Or update the S80 ... you’ve posted in the S90 XS/S70 XS Forum... S-series synths from this century! lol
Your S80 simply pre-dates the feature.

Hope that helps.

Thanks so much for the detailed reply!

1) In regards to the sustain port itself not being able to interpret the FC3A pedal, that's what I was afraid of. Also explains why I couldn't find any info about half-pedaling in the manual, ha. Good to know for sure, though. (I do have a basic on/off sustain switch that I can use as a backup, and have been using for years.)

2) I'm wondering if there are any workarounds for the polarity issue when the FC3A is plugged into the Foot Controller port... this seems like it could be a solution? https://beatbars.com/en/trs-extension-reversed.html Currently I have the FC3A "working" via the foot controller port with a full range of midi data, it's just reversed.

3) Another alternative could be something like this? https://beatbars.com/en/expression-to-midi.html (It seems to have an option called "reversed" which I'm assuming reverses the polarity.) I'm assuming that even though this would plug directly into my computer, I'd still be able to play the S80 and use the FC3A (through this device) at the same time within Logic Pro?

4) And if all else fails, yes I will probably end up getting a keyboard from this millennium, good idea haha.

Thanks again!

I really liked the S80 when I first thought of upgrading my keyboard rig back when. If I had followed through with the S80 purchase vs. picking up a second hand MO6 - I may still be playing on an S80 too.

I'd hold on to the FC3A for some future keyboard that's compatible with it (or sell/trade it - if you're more short-term) and get an FC4A that is compatible with the S80. Not mess with duct tape and glue to get the FC3A to work.

Yeah I'll probably keep the FC3A for future use since I know I'll ultimately want it. I think I could theoretically make it work in the Foot Controller port in the S80 if I get a polarity reverse dongle, but I want to be able to assign the FC7 to Modulation. I'm pretty sure that can only be done via the Foot Controller port because the Foot Volume port only lets you choose Volume or Expression, so I couldn't use both the FC3A and FC7 the way I want to. Will have to pick and choose.

Anyway, thanks for the advice and help!