--- Adrian Graham <Adrian.Graham(a)corporatemicrosystems.com> wrote: Not exactly. Right consequence, wrong cause. Q22 slots are where you put your CPU, peripherals, etc. If you have Qbus memory, it's where you put your memory cards. Empty slots between the CPU (first card) and the last card must have a bus grant in them or the devices behind the gap won't be detected when they make an interrupt. You can put cards that do not interrupt all the way at the back with multiple gaps and no harm done. Think of it like a bucket-brigade when someone steps out of line - there's no path from the back to the front. CD slots are special. They are wired up so the bottom face of one card is connected to the top face of the next card. Prior to the Qbus, when DEC made a multiple-card peripheral (RK8E, for example), there were H851 connector blocks to provide interconnection. This was not an ideal solution, especially given the risks that a customer might install a card upside down if it had 4 backplane fingers on each end and no handle. The first peripheral I worked with that required a CD backplane was the RLV11 - two cards, one always above the other (so they were properly interconnected) and no external interconnect was required (H851, ribbon cable, etc.) In more modern boxes, BA-23, for example, they have Q22/CD for the first three slots either for the CPU (uVAX-I) or the CPU and memory (uVAX-II, etc.) since late-model Qbus CPUs have PMI memory (Private Memory Inter- connect) and do not sit on the Qbus. CD slots never require grant cards. Personally, I have a bunch of 11/23-era (KDF-11) stuff, including a few BA-11N boxes - 9 Qbus slots next to 9 CD slots. It's handy because back when I got this stuff, an RLV11 was $100 and an RLV12 was $900 (since it worked in MicroVAXen, etc). By the time the KDA-50 was created, CD slots were no longer the answer and there's a ribbon cable to connect the two boards. Hope this clarifies the issue. CD slots don't require a grant card, but that's because they are outside the grant chain. -ethan __________________________________________________ Do You Yahoo!? Send your FREE holiday greetings online! http://greetings.yahoo.com