Shared Direction pins vs dedicated Direction pins

re: dedicated Direction & Step pins need 2*N output lines
vs shared Direction & dedicated Step pins need N+1 output lines
(N is number of axes), thus saving output pins which are limited
in number on a parallel port, and hence often a constraint.

Chris L wrote:

> >>It would appear to be for some type of "single axis at a time positioning"

Doug wrote:

> >Absolutely not!
> >This is a general scheme for lathes, mills, routers and hexapods

> Maybe by a "hand cranker".... but, On a CNC mill or Router ?? Now I am
> lost..... Even on my simple 3 axis router, I assure you I can and do
> have more than one motor turning, and in opposite directions at any
> given time, even for simple 2d profiles.

Hi Chris:

Yup, lots of motors turning simultaneously at different speeds,
accelerations and directions, thats how S&D motor drivers work
(as you well know!).

Yes, I think you are missing something obvious.... the Step
signals to the motor drivers are of the order of 1 uS (1 millionth
of a second). Apparent smooth motor motion results from emitting
one of these pulses (say 20,000 times/sec or once every 50 millionths).

In other words, you could have many (50 channels in this example)
sharing a common direction pin, and neither the motors nor the human
would be able to detect any measurable difference. However the Geckos
(much as I like them) have a hold-time constraint on the Direction line
which precludes their use in a "shared-direction" scheme.

<more written offline to Chris>....

Doug Fortune
http://www.cncKITS.com