Had this baby for 20 years at this point. Some rubber rot on the cables, but other than that, the set's in great shape.
RCX 1.0 with DC-in port. Serial tower. Fresh batteries in tower and RCX.
I have RIS 2.0 installed on a WinXP laptop with a serial port. Tower works and shows green light. The entire 2500-step firmware update process happens just fine (from the RIS settings menu). I've even successfully completed the IR range test, where it makes the RCX beep as long as it's in IR range.
After that, I get periodic "Trouble communicating with the RCX" messages. Downloading a program often takes 2-3 retries because of this. This seems like an okay work-around, but then other times, it just can never complete a download at all.
I just tried NQC today, and it downloads programs every time, reliably. That implies that this isn't an RCX problem or a tower problem. Some kind of RIS software problem.
I've seen discussions of other compatibility issues with newer Windows, but I'm not having those specific issues. Just really flaky program downloading with lots of retries. But the strange thing is that the firmware downloads just fine.... and that's HUGE.
As for why I don't just use NQC.... I'm trying to get this working for my kids. They've never done anything like this before, and the RIS interface is perfect for their skill level.
Update, More details:
Green light on serial tower comes on each time I try to download a program, so nothing else is blocking the serial port. Though you're right that NQC cannot connect to the port if RIS is running. I've tried both near and far range on the tower. Replaced batteries a bunch of times.
The only other bit of behavior that I forgot to mention is that when trying to DL a program, the green light on the tower comes on, and then sometimes the little "receiving" icon on the RCX comes on, but just for a moment. Other times, the receiving icon never comes on at all. Then, after several retries, the receiving icon on the RCX comes on, stays on, and the little progress "dots" on the RCX fill up.