A quick note which I learn from Rockwell TechConnect engineer.
You can reset the Allen-Bradley SLC-500 by shorting the VBB to the Ground.
Basically these are the steps.
1) Power down the SLC-500
2) Pull out the CPU from the backpanel.
3) flip the CPU card on the back and look for VBB and GND on the CPU. VBB and Ground is as shown below.
4) Use a short wire or a screw driver to short GND and the lead marked as VBB. This resets the CPU.
I hope it help you.