Thanks Jazz. Apologies, that came across wrong, ill try to explain. I wanted any fault signal to disable all movement and I could not figure out an easy way to combine the 4 stepper driver fault signals into one so I got them to switch 4 small relays and then wired the relay switches in serial so if any one opens it cuts my 5v enable circuit etc.

There are probably better solutions but I replaced those 4 relays with the arduino using 4 inputs and 1 output. Arduino Inputs connected to stepper driver fault outputs and the 1 output connected to all driver enables and spindle relay. The few lines of code check the inputs and if any change is detected it takes the output pin low disabling all drivers and switch the spindle off. Ive tested this to death and it works like a charm.

Not sure I understand what you mean about the spare IOs. I do have spare inputs available on my controller but i dont want to rely on mach3 alone to stop everything or is that not what you mean?