Just an update on all of the above. Step/Direction issue resolved. Trying today to connect the step and direction from a 5V BoB with NO Line Driver. So just single-ended 0-5V. I appreciate that this is not too noise immune and it may not work at all. There are two 51ohm resistors either side of the optocoupler input diode inside the Delta ASDA-B2 drive. If you do as they say with the line driver, that is a 10V differential across these resistors in series. Gives about 0.04901A, or 49mA of current by their (Delta's) design to switch the optocoupler. HOWEVER... we do not have access to the tech. specs. for the optocoupler which they use and it is my suspicion that using single-sided input (0-5V) may just work. It is obviously not ideal, but it will still give 24.5mA across the optocoupler diode which may still be enough to switch it on. The only place I see a problem with this is that at high speeds there won't be good step clarity and it may result in lost steps to the drives. (To the drives, not to the motors, which are obviously closed-loop). So, in summary, I will test this now and my guess is that provided we stay below 100kHz pulse trains (200kHz is the spec.) that it should be ok. Will post updates shortly. (I note again that you will need EXCELLENT noise shielding to do this and not lose steps.)