Re: 5-axis machine controller

[Jon Elson <jmelson-at-artsci.wustl.edu>]

"D.F.S." wrote:

> > I am a little cornfused as to some of the other responses about "phase
> > driving with EMC. I was under the impression that there is a 2 bit phase
> > output meant to drive a particular machine that had that type of driver.
>
> That is sure what I get from reading it as well.
> It is a driver for specific hardware.
> BUT, something I never did see mentioned is if I'm reading it correctly you
> should be able to simply stick a pair of 74139 TTL Dual 2-to-4 Decoder Chips
> on the outputs and off you go. The 2 bits select ONE of 4 outputs wired
> to each of 4 phase driving transistors and you are done.
>
> You can't drive more than one phase at a time and so won't be able to 1/2 step
> but this is almost as simple as directly driving the 4 phases from 4 bits.
> It would allow twice as many motors to be driven from a regular parallel port.

The output of what we have now for phase drive in EMC works like
this, for going in one direction (the other direction would go through the
table backwards) :

                  EMC outputs     Bipolar Drive        Unipolar Drive
state    ph A  ph B   ph A  ph B   A1  A2  B1  B2
  1        1     0     +      -    on  off  off on
  2        0     0     -      -    off on   off on
  3        0     1     -      +    off on   on  off
  4        1     1     +      +    on  off  on  off

In this table, for Bipolar Drive, the + and -
indicate which polarity voltage is applied to each
stepper phase winding.  In the Unipolar Drive, on
and off refer to the state of the transistors, and
the A1 and A2 refer to the transistors on the opposite
ends of the same phase, and B1, B2 are the other
phase.

So, you see that 2 bits can control the 4 transistors
of a unipolar stepper driver pretty easily.  A1 and
A2 are never on at the same time, and both are never
off at the same time (unless that would be commanded by
a machine-off or E-stop signal).

Jon