Jerome Fine replies: That would explain the situation since no one else seems to have ever seen a pure Q22 backplane for a BA23. Since I can use a BA23 box with a PDP-11/73 with BOTH normal memory below the CPU and PMI memory above the CPU (actually, it can even be the same memory), the uVAX-I should also be able to do so as well. However, for a pure Q22 backplane will all slots wired AB /AB (I presume that is what you really meant to write), PMI memory would not be possible for the PDP-11/83. I presume that would also preclude a uVAX-II which I assume also needs AB / CD slots as well. On the other hand, if all the slots were wired as AB / CD as in an BA213 box (is BA213 correct - that would be for a uVAX-III) which has only AB / CD, the PDP-11/73 should also work along with the PDP-11/83 although I have never hear of anyone using these boxes for PDP-11 CPUs since they require different boards which likely are not available to be used with the PDP-11 in most cases. Tony, do you have a UVAX-III box or can anyone else comment? Note that a BA11-S box is a 22 bit 4 * 9 backplane and wired AB / CD for all 9 Qbus slots. One of these years I will try using a PDP-11/83 and PMI memory to test the situation. Sincerely yours, Jerome Fine -- If you attempted to send a reply and the original e-mail address has been discontinued due a high volume of junk e-mail, then the semi-permanent e-mail address can be obtained by replacing the four characters preceding the 'at' with the four digits of the current year.