booting rt11 v5.1 on an 11/45

The 11/45 will boot xxdp+ v1, but won't boot RT11 v5.1. Upon booting the RT11 v5.1 RL02 pack, the disk flashes for about 5-8 seconds, then the processor just halts. First question which I didn't see in the RT11 5x docs... what exactly would the hardware requirements be for RT11 5.1 on a 11/45? Is it expecting a minimum of memory? Certain I/O devices at certain locations? I tried both a real DEC RT11 v5.1 RL02 dist. pack, and also was able to gen one from a 5.1 image on a scratch pack with vtserver. The only thing I can think of (and another listmember point to as well) is a problem with interrupts. But I've no idea how exactly to go about troubleshooting it, not being a DEC expert like others here :) I've gone back and forth between using 8K core, 16K core, or 128k unibus memory. Similar results. My copy of xxdp+ seems a bit old, so most diags I have seem to have older names. Any thoughts are most appreciated! Jay West