I have recently become the proud owner of a PDP-11/04 with a set of 11/34 cards (M8266 & M8265) This particular 11/04 has some stuff on an extension backplane that I don't understand: PDP11/04-LH configuration UNIBUS layout: A B C D E F M7263 M7263 M7263 M7263 M7263 M7263 1 M7891 M7891 M7891 M7891 M7891 M7891 2 blank blank blank GRANT blank blank 3 M9312 M9312 M7859 M7859 M7859 M7859 4 M7762 M7762 M7762 M7762 M7762 M7762 5 M7856 M7856 M7856 M7856 M7856 M7856 6 blank blank blank GRANT blank blank 7 M7856 M7856 M7856 M7856 M7856 M7856 8 M920 M920 blank GRANT blank blank 9 ------------------------------------10 M920 M920 M7219 M7219 M7219 M7219 11 (- M9688A-) M208 M208 M7821 M796 12 PWR M205 M611 M611 M112 M113 13 M9302 M9302 blank blank M116 M239 14 The M9688A is a very short card - titled DR11-B Test Board The stuff in slots 11-14 is what I have no idea about. Anyway - I also have a M8265 / M8266 pair that I would like to use to upgrade to 11/34A if possible. To do that - I suspect that the config changes to: A B C D E F M8266 M8266 M8266 M8266 M8266 M8266 1 M8265 M8265 M8265 M8265 M8265 M8265 2 M7891 M7891 M7891 M7891 M7891 M7891 3 M9312 M9312 M7859 M7859 M7859 M7859 4 M7762 M7762 M7762 M7762 M7762 M7762 5 M7856 M7856 M7856 M7856 M7856 M7856 6 blank blank blank GRANT blank blank 7 M7856 M7856 M7856 M7856 M7856 M7856 8 M9302 M9302 blank blank M116 M239 9 ------------------------------------10 The removal of the M920 should mean that the extension is effectively disconnected. blank blank M7219 M7219 M7219 M7219 11 (- M9688A-) M208 M208 M7821 M796 12 PWR M205 M611 M611 M112 M113 13 M9302 M9302 blank blank M116 M239 14 Any help from an expert would be appreciated. Doug