Let's look at this one first, this is probably the easier to solve. Ah. That's D11. :-) So that's likely in the memory (although I suppose it could be the CPU, _somehow_). It sounds like there's a latch somewhere in the output path (because it affects all locations right away, not just once you've written to them) that's getting set one way or the other, and and then, won't change. I suspect the problem is with the flop for that bit, not in the circuitry that's clearing/clocking that flop, since it only affects that one bit. Looking at the MS11-J prints, there is indeed a '174 latch in the output path; the one for D11 is in E30 (input pin 14, output pin 15). You might want to throw a scope on it, and see if it is indeed acting consistently with the symptoms (to make sure this is actually the cause). Although why it can be either set or reset with a write, but freezes in one state for reads, is puzzling. I'd suspect control circuitry, but it's only that one bit. I don't think it can be something on the input side, because the memory chips have input and output on separate pins, so if something was hanging on the intput side, it shouldn't make it through the chip. Yeah, sounds like the CPU is halting. It's probably going to take a logic analyzer to figure out what's going wrong. Too bad that machine doesn't have a real front console, that would probably let you figure out what the problem is. Noel