Getting a PDP-11/05, RX11, RX01 working

I've made some good progress with getting my 11/05 back to full working condition. The stuck bit I had earlier this month I resolved by replacing the sense/inhibit board. Thanks to everyone who advised me on this. I've also got RS232 working on the 11/05s on board console port. After a hunt through many boxes of accumulated junk I came across a little box with an RS232 port on one side and a berg connector on the other. Within the box was a crystal and some level converters. This was exactly what I needed, providing a stable external clock (9600 baud) and using the TTL levels available on the berg connector to provide RS232. Next I acquired an M9301-YF which I fitted to the terminator slot closest to the CPU and booted into its on-board console. All worked well here and I could examine and deposit values into memory via it. Then in went an RX11 (into the SBC slot) which I hooked up to a RX02 with its switches set to make it behave as an RX01. When I tried to boot by typing DX into the console after about half a second of running the machine halts and displays a PC of 4 (IO Trap I believe). If I repeat with the power to the RX01 drive switched off it continues to run until the RX01 is powered on at which point it halts. The RX02 I know to work (as an RX02 at least) as I've connected it to a QBUS machine with RXV211 to confirm Anyone have any thoughts as to what is going wrong? One possibility that occurred to me is that the SBC slot is missing the NPR signal. I have read that this is necessary for an RX11 to work. How do I check if that is present and how do I put it in if its missing? Thanks, Toby -- This message has been scanned for viruses and dangerous content by MailScanner, and is believed to be clean.