Cessna ATC Program Module (no swapping of rack pins needed!!!!)
All that is needed is to set the dip switches to the binary code for the aircraft. Cessna's PDF leaves you to draw some extra lines on the right of chart for the #2 transponder, but using the example on the left helps out. Don't use the image..... download the PDF below.
I've also heard a similar module is used for the ELT.