Here is how I wired mine to the C10
There is a jumper on the C10 to set the COM terminal to 5V or GND. I set mind to GND. Also mine is the C10 revision 8.2 and the inputs have 4.7K ohm pull own resistors. I had to cut them out and use pull up resistors on my inputs. Also, this is the very first revision of the G250 so it required a 100 ohm resistor on the logic ground ... I don't think it's required any more.
I am attacking a higher-res image if you need it.