[Simh] unix system 7 question

[Tim Newsham]

Hi, First of all, thank you for your project!  Its nice to see
people working to save our rich history.  I am very grateful
for your efforts.

That said, I was slightly disappointed at the level of documentation.
I had no idea how to use the thing and was having a hard time
finding any information on how to boot some of the software packs.
After some web searches I was able to find information about
how to boot the unix v5, v6 and v7 packages.  It would be very
useful if the page that has the software kits links also had
a brief description of how to get the distribution going in
simh.

Now on to my question.  I am able to boot the v5 distribution
fine using notes I found online (ommitting the "set cpu 18b"
directive which seems not to be present in the current release).
With v7 I am having less luck:

  PDP-11 simulator V3.2-0
  sim> set tto 7b
  sim> attach rl unix_v7_rl.dsk
  sim> boot rl
  @boot
  New Boot, known devices are hp ht rk rl rp tm vt
  : rl(0,0)rl2unix
  ka6 = 2225
  aps = 141746
  pc = 31774 ps = 30004
  trap type 0

  Simulation stopped, PC: 063132 (CMP #57054,R4)

I'm getting a trap when trying to boot the image.  The instructions
I'm following are roughly taken from:
    http://wandel.ca/homepage/unixdemo.html

Whats the error here?  Has the emulator changed?  Has the image
changed?  Is there an argument I need to set that I am not setting?

Tim N.