Oooh, thanks ever so much. Not sure how I missed that when I looked on BitSavers for RH11 stuff! Very illuminating - eventually! The M7294-YA seems to be a manual ECO to the M7294; there's a detailed rework list on page 6 of the PDF. I'm still trying to work out what the changes do. E66 is a 'component carrier' header, so it seems like in part the ECO adds a bunch of option-controlling jumpers there (see pg. 2 for a table of what they do). The main thing, though, seems to be the addition of E22, a 74191 binary up/down counter, on page DBCA (pg 10 of the PDF). It seems to modify the operation of 'Bus Hog' mode - maybe to do 16-cycle bursts? (All the bit inputs and outputs are unused; only the Max/Min output - pin 12 - is connected to anything.) That would make sense; with UNIBUS A and B tied together, the original Bus Hog (below) would lock out the CPU from the RH11 until the end of the transfer. Actually, though, even without the cross-connect, having the RH11 going flat out Bus Hog might lock out the CPU from the _KS10 memory_... The RH11-AB has Bus Hog too; see Section 4.12.10, "BUS HOG Mode", pg. 4-22 (59 of the PDF) in "RH11-AB Option Description" for details; it "hold[s] the Unibus ... until the required number of words have been transferred". Noel