Finally found a moment to take a gander at that: The KDJ11-E memory seems to be parity only, not ECC. There's a Parity CSR at 17772100, which will tell us if a parity error was detected in the main (on-board) memory. There's also a Memory System Error Register at 17777744, but that's only there for backward compatability, and I'm not sure it will tell us much. It might also be good to look at the system CSR at 17777526, which will tell us if the firmware has set the memory size to how much memory is actually on the machine. There does not appear, alas, to be a register that indicates _where_ the parity error happened. However, if it's happening regularly (and still, as in 'problem disappeared in the process of diagnosis') we should be able to work out where it is. Although I'm still somewhat astonished that a persistent parity error wasn't picked up by the self-test on power-on. In theory, you should be able to plug most QBUS boards into those slots (although if there are QBUS cards that barf in a Q/CD slot, one couldn't use them - I can't recall if any such exist, though), but... no device board is going to work properly since the two QBUS bus grant lines (BDMG and BIAK) will be wired past those slots, directly from the CPU to the KTJ11. On the 11/84 backplane, there are a couple of jumpers that _can_ send the bus grants through the 'QBUS' slots on the 11/84 backplane (so _in theory_ one should be able to plus QBUS devices into those slots, and have them function correctly, when those jumpers are set appropriately, but we have yet to confirm that), but I don't know if the /94 backplane has anything similar. Alas, there are no 11/94 prints online (that I know of) to confirm that. Excellent! The more known good, working components we have to play with, the better! Noel