I don't use the UB1 and its not very clear on your JPG diagram
but I don't see how your series connection of the stepper driver enable inputs will work
each of the drivers input opto-isolators (step, direction & enable) will have a current limiting resistor of about 300 ohms to limit the LED current (to about 10 mA) when you connect 5V across the input terminals
the enable input of all the stepper driver I have looked at would be better called the disable input !!
( with no connection the driver is enabled , connecting 5V to the enable input will disable the driver )
what stepper drivers are you using ?
John