I used to be fascinated with the IIsi (hence my handle). I have every piece of information (just about) that you can have relating to these machines, from the pinouts of all the chips inside them to the locations of different things in the ROM. You name it, if it deals with a IIsi, I probably have it or can get it. I did some digging and sure enough, I have a document (which used to be easy to get and some of you have probably seen before) that details the video pinouts, signaling, blanking timing, etc of the RamBasedVideo graphics chip/interrupt controller in this box. Based on the description, you could easily play around with the video sense signals. PM me your email address and I'll send it to you.