What I'm trying to do is very much like you configuration - my real 11/53 has a KDA50 with 2xRF70 drives and the secondary MSCP controller is a SQ703. You can demonstrate my problem with simh, though - build a bootable XXDP+ v2.5 on a RD52 image, and then do this... sim> show cpu CPU, 11/53, NOCIS, idle disabled, autoconfiguration on, 3072KB sim> show rq RQ, address=17772150-17772153*, no vector, 2 units RQ0, 159MB, attached to one.rd54, write enabled, RD54 RQ1, 159MB, attached to two.rd54, write enabled, RD54 sim> show rqb RQB, address=17760334-17760337*, no vector, 30MB, attached to xxdp25.rd52, write enabled, RD52 sim> b rqb0 HALT instruction, PC: 016146 (CLR (R1)) If you attach xxdp25.rd52 to rq0, it'll boot fine - sim> att rq0 xxdp25.rd52 sim> b rq0 BOOTING UP XXDP-XM EXTENDED MONITOR ... etc, etc .... I suspect you have to patch something in the XXDP+ image with UPDAT to get it to talk to the secondary CSR, but I don't know the details. Thanks, Bob