At 05:31 PM 2/1/02 -0500, you wrote: I don't know that much about Qbus, but I beleive that there are 16bit, 18bit, and 22bit versios of the backplane and perhaps while some boards may be backwards compatible, it seems unlikely that the older 16bit boards are forward compatible. Again, I know mostly about Unibus, so I am guessing that the OMNIBUS is not as advanced as the Unibus, which has 4 levels of bus priority, with devices at the same level responding first if they are closest to. This is done in a chained fashion, so if a card is not present, then either a grant card must provide the continuity or wires must be added to the backplane. Otherwise the signals will not make it past the open slot. There are both 4 and 9 slot Unibus backplanes. Also, most processor backplanes have at least one Unibus slot in them to provide room for a DL11 console card (or similar). The Unibus comes into the A and B (top two) connectors of these backplanes and leaves or is terminated in the last A and B (top two) connectors. Hex cards can be plugged into the slots which are not on the ends. Quad cards can be plugged into the end slots. In a 9 slot backplane, only 2 hex cards can be used. First and last are as viewed from the card side, not the wire wrap side, with the power wires at the top, and the first on the right, which is the side that the CPU is placed on. 4321 987654321 +----+ +---------+ |DD11| |processor| A \ | | |backplane| | |====| |=========| | Unibus | | | | B | signals | | | | / |====| |=========| | | | | C | | | | |====| |=========| | | | | D | | | | |====| |=========| | | | | E | | | | |====| |=========| | | | | F | | | | +----+ +---------+ Viewed from side cards plug into. --tom