Are you inquiring about hooking a modem directly to the radio? I don't recall any such port on the IC r8500. You can however use a cable (db 25 to db9 non null if memory serves correctly) to interface the radio to your computer for remote control and programming. Pretty old software however so I'm not sure how well it would play with modern computers.
We run a small cable system at work and have a few hundred cable modems on it. The guy who used to run it was an amateur radio operator. He did a lot of research on the actual modems to see if they could be tricked into doing something other than running data over a cable system.
They use space below the lowest TV channel for data communications. Nothing useful, and you won't pick up anything other than a bunch of noise. I also don't know for sure what signal levels are coming out of there, but I wouldn't connect the RF side of a cable modem up to a nice radio like an 8500 without some attenuation in there just to be safe.
Connecting your 8500 direct to the cable system might get you something. Most systems have gone digital, but you might get some FM stations on there. It's been a long time since I had cable TV, but back then they ran the FM radio stations on there. That was well back in the 90's and things have likely changed.
I remember years ago connecting a scanner or receiver to a cable TV line and being able to receive some stuff, like TV audio and FM radio carried over cable. I haven't tried this in a couple decades however, and I suspect that it wouldn't work due to the cable being all digital here.