Generally you will need the C41 board not the C6 for spindle control. why did you do that?

c41
Inputs a PWM and outputs an analog 0-10VDC.

C6
• Inputs a frequency (step) and outputs an analog 0-10VDC.




Read the manuals how to connect them:
http://cnc4pc.com/Tech_Docs/C6R6%20and%20Huanyang.pdf

as an answer to your question from the manual:
Make sure the grounds of the analog circuit are keep
isolated from the logic (+5vdc). Use a multimeter to confirm there is no
continuity between the two grounds before applying power.


Dont be in a rush.