These ones:
http://www.mechapro.de/hem545.html ...german... ttl/optocoupler, powerd with 50VDC, good for 5Amps. Powering 1 Stepper each.
! for the y, one for z and 2 for the x. Thru a relais matrix I can switch between 2 mills.
I use those for years now. Never had any issue.
Hope my Multimeterscope is good enough otherwise I will have to carry my big one down to the basement. will take a while....