JP 1482 Spindle Over current signal output

If the drive outputs a digital signal, then you could connect that, through HAL, to the pin halui.estop.activate
linuxcnc.org/docs/2.7/html/man/man1/halui.1.html

That might be an over-reaction. Perhaps the feed-hold in motion is what you want.
linuxcnc.org/docs/2.7/html/man/man9/motion.9.html

doityourselfer wrote:

I attempted to connect an output to the pin marked "sp fb sig" on the integrated circuit. I induced a fault but it wasn't detected.

Are you trying to force a spindle fault from the software, or have the software detect a spindle fault?

In the latter case you would need to connect an _input_ to the sp_fb_sig and then would need to check that the pin changed state.
It is possible that the input pin is pulled high, and so the sp_fb_sig going high has no effect on the input state.

You should be able to detect the state with a multimeter. Also, bear in mind that unless the spindle logic and IO logic share a GND it is unlikely that signals will be reliably measured. You might not want to do that, in which case a second Opto-Isolator driven from the opto-inputs on the board might be appropriate. (But only bother with this if it is necessary, as an alternative to a relay or whatever)