YooCNC T62-3AX Reverse Engineering

I have been planning to add limit switches and spindle control ever since I got my 3020 CNC a while ago. Many people have done it, and there are a few pages that show the "idea" of how to do it. It all boils down to a few unused pins on the T62-3AX board. This board I believe is exclusive to the 3020 CNCs (and maybe 3040) as the larger ones use separate motor driver boards and a breakout board.

I previously did a limited post on the T62-3AX board on my blog, but I only showed a limited schematic here:
YOOCNC T62-3AX CNC Controller for 3020T

I feel the first step should be a solid and complete reverse engineering of the existing board to make sure we fully utilize its capabilities. This will also serve as a great reference to those needing to make repairs or wanted to further understand its operation. I will be creating a Fritzing file that will have a PCB layout and schematic of the board. I will upload it as I work on it, so there will be versions that are incomplete or missing components. If you find any faults with it or want to help add, please do and send me the changes so I can upload them here.

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General schematic screenshot from Fritzing (download link is below):

 

Please note, some components may be missing or mislabeled.

Current Fritzing file (created in 0.92):
YooCNC_T62_3AX_v04.fzz (Last Updated May 11th, 2015)

What I have learned from drawing up this complete schematic of the CNC driver board:
1) Unused pins, list to come.

2) Unused opto-couplers & header pins, idea for limit switches.

3) Unpopulated positions for DIP switches permit changing of stepper driver presets. Details to come...

4) Generally well designed except for missing A-axis control. My current focus is on adding an A-axis to control the feed stepper for my 3D printer attachment I've purchased.