slaved stepper problems

[Jonathan]

Have you slaved it in mach3, so the drivers are connected to seperate pins (4 in total) of the parallel port, or is it one step/dir signal going to two drivers in parallel? If it's the latter that would explain it as the parallel port can't source much current, so with two drivers the voltage will drop.

[Gary]

yes as jonathan has said, putting one output in parallel is not a good idea.
if it has its own output from the parallel port than it is just down to tollerance.
the driver that has the problems is just slighly out of spec or the others just have a higher tollerance.
just power the opto up with a good +5V and problem solved.
Or get a breakout board that buffers the outputs.
Simples...

[johngoodrich]

yes it is slaved in mach3

[boldford]

I know I'm late coming in with this but if you really must run the same axis on two stepper motors I'd be inclined to ensure they are identical bipolar type and connect them in series.