Thanks Jim.
I think so. I did the same maths. I agree that is the first two pulses. I think that this occur in one direction but not in the other.
I will do your suggestion.

Nemo

Finally I fix the problem. I think.
After many electronics test I found the problem, which was the Optocoupler IC from step motors drivers.
After I changed all optocouplers from drivers the problem disappeared.
Great! Now my CNC is running fine and accurate.
I think that one signal probable "DIR" was retarded by the old optocoupler then the step motor driver loses "step pulses".
I don't have a scope and I think that with one , this kind of problem would be more easily set on.
Thanks Jim
Thanks All.

Nemoc