MicroVAX II - TK50 problem

On Thu, 10 Mar 2005 14:18:59 -0700, willisjo at zianet.com <willisjo at zianet.com> wrote: I commented elsewhere, but if you literally have this arrangement (one memory card), you aren't passing grant on the right side of slots 4 and 5. If this "memory picture" is not entirely accurate, you might still have a grant issue. The RQDX3 does not have to be the last card on the bus (unlike the RQDX1, which _does_), but it was customary to put it there, partially due to the cable which comes up from the bottom of the card cage and might cause interference issues with some other option cards' cables. Also, it has a command queue, etc., sufficiently long that it won't starve at the end of the bus, so it's safest to put it there. Normally, communications cards and tape cards are considered time-critical because of the nature of tape moving past heads and bits passing down the wire cause extra work if they have to be re-done. Disk interfaces of this era are sufficently complex that they don't need to be at the front of the bus. Note that this was more of an issue in the days before the uVAX-II when peripherals had shorter internal buffers and needed to be prioritized or they might suffer bus starvation. It was also common to put an M9047 grant card in the AB part of slot 3 as a place-holder for a memory option later. That prevented having to completely rejigger all the cards, which might be required if one had a dual-height option (like a DEQNA or TQK50) in slot 3. Given that Qbus grant cards were in no way as common as Unibus grant cards, it wasn't a universal practice, but it's still a good idea if one has a spare grant card lying around. Hope this clears up some of the voodoo of Qbus configuration. -ethan