MicroVAX help

<Allison wrote: <> The uVAX-II <> offered 1meg of ram on card, FPU and a faster memory interface(PMI) <> along with denser 1mb, 2mb, and 4mb (and later 8/16mb) cards. This made <> a 5mb microvax-II possible in two cards instead of 7 using uVAX-I! < <AFAIK the MicroVAX I can't do 5MB no matter how many cards you have. It's <a pure QBus machine; all memory lives on the QBus and there is no <scatter/gather map to allow it to get to more memory than the QBus can <address, which is 4MB. Right you are. But in the context of what I said, you see that 5mb uVAX-I was difficult for power and bus space reasons as well... the fact that the microvax-1 didn't not have PMI(over the top memory connect) and Q22 could not address more than 4mb was just one more limitation. What happend in real world cases is the total load on the power supply was way to great or you ran out of slots! The one I ran for a while back in 86 at DEC was 2meg running microVMS. That box was full! 2 quads for the cpu, 2 quads for the memory, 1 quad DHV11, 1 quad RQDX2, 1 dual for tk50, 1 dual for DEQNA, 1 RD53. The power supply on that one ran at 95%. It was slow but I was one of the few with a vax under my desk! ONE ITEM: BA23 boxes have a cable from the power supply to the backplane. There are two different ones. There is one with variable length wires that looks neat and tidy making the bend. That cable is bad news! It was ECOed back in the late 80s as the +5 and ground are parallel lines! The cable had different length leads and the shorter ones having lower resistance would hog all the current. This would cause a serious overload of the connector pins with resulting is possible destruction of the power supply and backplane. This would occur at less than rated load! The correct cable has ALL of the wires the same length to circumvent this problem. The revised cable was tested at 200+% of rated load and did not exhibit the failure mode. So if you playing with MicroPDP-11s or VAXen in the ba23 box check for that flaky cable and replace it if possible or at least check it for signs of overheating at either connector. It's molex pins and if not damaged the connector bodies can be reused. Allison