And it does! Quite a few values come back wrong, when ECC is disabled - I'm going to guess about 25% of the time. (Out of 0-020, 4 were wrong.) And it is not board-dependent - two different boards give incorrect read data for the same write values!! And the ones that were OK were OK on both. (And it doesn't appear, from a bit of spot-testing, to be address-dependent.) This is _very_ strange. There's nothing in the manual about 'disabling ECC causes incorrect data to be returned' that I could see. I wonder if the board is storing wrong values a _lot_, and the ECC is normally catching them? (Maybe DEC did it this way to test the ECC hardware all the time, and quickly catch failing ECC? But why doesn't the manual mention that?) One thing I noticed is that while I was doing the 'which bit goes in which chip' stuff, on some of the data lines, there was a lot of grup - some of it fairly long pulses, and some spikes that looked like they might be hazard outputs. I wonder if they are part of the cause? I guess the next step is to set up a loop which stores one of the values which always gives a bad output, and see what the board is actually writing into the chip... Very, very strange! Noel