Err, think you mean RLV12 QBUS controller board, right? Anyway, you've already tried ("three controllers") swapping that out? If you have, perhaps there's a problem with a cable and/or the terminator? However, at this point, sub-system swapping is probably not going to be the best way to make progress. I think you're going to have to actually debug the problem; i.e. dive in and work out what's happening wrong, and why, and trace it back to the origin. Luckily, for this generation (and before), DEC produced wonderful technical manuals, which go into full detail of how the thing works. With that in hand, the investigative process is a lot easier; you don't have to work out how the thihg works by looking at the prints, it's all laid out in detail. And tech manuals for both the RL02 drive and RLV12 controller (EK-RLV12-TD-001) are available online, as are the prints (MP01282 for the RLV12). So, I'd sit down with the RLV12 tech man and read through the section on the drive<->controller bus (section 3.3). Then start looking at what the two are saying to each other on the bus, and figure out what's actually going wrong. further. I know this sounds like it would be time-consuming, but when you think about the time/energy you've already put into swapping things around, chasing this problem, it won't be. Noel