The board makes use of the XRDY line on the S-100 bus. It also controls the RUN signal (although not needed on a IMSAI for example) so it would also work with systems that do not have a front panel.

Features include:
    Address Bus - LED display
    Data Bus - LED Display
    Status Bus - LED Display
    Auxiliary Bus LEDs - User Configurable
    Single-Step Program Control
    Hardware Breakpoint - For debugging software
    Voltage Test Points.
 
The schematic for this board can be obtained here.
If you wish to obtain a board try here.