I have one of the generic Chinese breakout boards like this
https://www.ebay.co.uk/itm/274524902217
When it is powered up but not connected to the PC, the input pins go high. This is fine except that when pin 17 goes high the relay switches on, and hence the spindle. So if the PC fails while the CNC is switched on, the spindle will start. I don't think this is safe or convenient. I assume that the BOB has pullup resistors on the inputs. Seems to me I have two options. One is to desolder the resistor for pin 17 and replace with a pulldown resistor. But I don't know where the pull up resistor is, or whether there is one for each input pin. The other is to try to pull the pin low with a lower value resistor than the pull-up, in the hope of bringing the voltage below the ON threshhold, while still allowing the PC to pull it high when needed.
Thoughts?