Quote Originally Posted by AndyUK View Post

My thoughts were, that when the output is off (i.e. safety circuit broken), no current can flow through the enable optocoupler, so its a logic low. When the output is on current can flow, logic high, drive enabled.
I haven't ploughed my way through the UB1 manual, but in general terms it looks like the internal "switch" in the UB1 shorts the pin to earth (in which case current can flow) or not, in which case no current flows. What might be causing confusion is that when the switch is on, current flows, and the output voltage on the pin is more-or-less at earth potential. When the switch is off, the output pin is high voltage (as the resistor and enable output path on the driver form a pull-up circuit). So is your logic the wrong way round? :Low and high usually refer to the voltage on the pin, not current flow.