Oups! you are right Ian: Mach3 is fine, and my recorder is wrong.
Reading again my code, I found I forgot to latch my positions counters (the ones that get incremented or decremented on PIN interrupts) before using them. As a result, by the time I output these to the serial port, they were altered by the interrupts.
Now I corrected the code, and everything is fine on both the parallel port and smoothstepper (just one pulse off on rare occasions, which is what one would expect).
I am very sorry about raising this false issue. I was too happy to find a beginning of solution for my still unexplained little wave patterns on the parts printed by the machine..

I will edit my first post, or maybe if a moderator is listening there, it might be better to just remove this all thread ?

Pierre
well, it seems I cannot edit my first post...

That is a shame Ian, would have loved another reason to beat Brian over the head with regarding V066

Can you not just beat him anyway? 

Pierre - no worries - thanks for the update and hope you find the reason for your waves.

 I do Ian, at every opportunity I get. Sadly it makes little difference though.

Hood