When I got my OSSC a little while back, I was able to get the 240p passthrough to work when my SNES was connected via SCART, but of course my TV did the upscaling and it looked like crap. Sadly, any mode other than normal passthrough would just die for me on my SNES.
Can’t really state the make and model of my setup since it’s fairly complicated. All of my consoles which have an HDMI output are plugged into a 5-port HDMI Auto-Switcher which then goes into my Denon A/V Receiver into my LG TV. So there are a bunch of things in the way which could have problems with the “non-standard” OSSC.
My LG TV is from 2007 and is a 42″ TV which I have set to PC Mode and that gives me a bunch of ways to modify how it looks. Still, the OSSC and NTSC SNES didn’t like each other.
I previously got this little generic SCART-to-HDMI adapter from Amazon when I JUST got my SNES. https://www.amazon.com/gp/product/B0177DG71S/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1
It was okay, but it would always stretch the SNES to full screen and seemed to have a bit of noticeable lag to it. So it wasn’t ideal. Later on, I replaced then with a cheap Gonbes 8220 Scaler and a scanline generator I designed in Eagle and etched out, and finally into a VGA to HDMI adapter. This was okay, but it was a TON of devices to pass through and was not very easy to store in my entertainment center.
As a last resort, I took the HDMI out from the OSSC and put it into the HDMI Input of the cheap converter I linked to above and connected the output to my HDMI Switch. It worked! EVERY signal except x5 now works great for my NTSC SNES and my setup. Even better is I cannot notice any lag from the SCART to HDMI adapter which makes me think the analog-to-digital conversion is where the cheap adapter is putting in all the lag.
So now, I have my OSSC sitting on top of the SCART-to-HDMI adapter that is acting as a signal smoother, so to speak.