I *suspect* that the whole right-hand-side of the Sigma box is CD-only. The only way to tell for sure is to pull the backplane and trace it out. This confirms my suspicion that the Sigma backplane you have has CD slots all the way down the right side (though it's not conclusive; it might only be CD for the first four slots.) If, indeed, it is CD all the way down, then putting a TQK50 in next to a memory card there can heavily confuse the memory mapping and testing. Yes, the early tests are memory bus testing and sizing. Well, if you're playing fully by the DEC rules you should swap the cards, but both the RQDX3 and TQK50 are smart heavily buffered devices, and in such a tiny configuration as you have I doubt there'd be any differences seen at all. OTOH, if you have 3 Q-bus backplanes with dozens of "stupid" (unbuffered) cards in each, it does eliminate a lot of heartache to play by the rules!