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.

Xybernetics Allen-Bradley SCL500 - Resetting Controller

3) flip the CPU card on the back and look for VBB and GND on the CPU. VBB and Ground is as shown below.

Xybernetics Allen-Bradley SCL500 - Resetting Controller
Xybernetics Allen-Bradley SCL500 - Resetting Controller

4) Use a short wire or a screw driver to short GND and the lead marked as VBB. This resets the CPU.

Xybernetics Allen-Bradley SCL500 - Resetting Controller

I hope it help you.

Cheers!