Re: First CNC project

Thanks Charles,

I can tell that I still have a lot to learn. Things have changed an
awful lot since my Navy days working on a Sonar system that had 1200
tubes and the power amplifiers were in water cooled cabinets.....

I really appreciate the information.

Do you know why some systems use a PLC between the PC and the tool?
Is it to control more I/O than can be accomplished with just the
parallel port? Or is it a power thing to control a drive with larger
stepper/servo motors?


--- In CAD_CAM_EDM_DRO@yahoogroups.com, Charles Knight <charles@i...>
wrote:

> >
> >
> >>Question #1. How do I interface between a PC and the stepper

drives?

> >>Question #2. Do I connect throught the parallel port? What

limitations

> >>exist?
> >>
>
> It took me a while before I "got it." Hopefully this will help.
>
> 1) There is a black box...a circuit which reads data from your
> computer, usually from the parallel port, and then tells the

stepper

> motors what to do. I have a chip on my desk which takes a "pulse"

as an

> input, and outputs whatever the next step sequence would be, for

the

> stepper motors. Most people will simply buy this box, prebuilt,

and

> just plug everything into it. Gecko makes such a box, and so do

several

> other companies. Masochist that I am...I want to build my own.
>
> 2) With a printer cable. Seriously! There is a layer of software
> inside your computer which will take the CAD data, and turn it into
> standardized instructions for a milling machine, called G-code.

Another

> layer of software reads the G-code, turns it into instructions for

your

> particular machine, and outputs it through the parallel port. You

tell

> it how many steps there are in an inch (200 steps per revolution

for my

> steppers, multiplied by 18 threads per inch for my leadscrew) to
> calibrate it, and then it does the calculations for

you...outputting the

> right number of steps for each axis, through the parallel port.
>
> The way it works is that the parallel port outputs 8 data bits,
> simultaneously...and it does it in binary code. It'll output

something

> that will look like 00100111, with each 0 or 1 being a voltage, or

lack

> of voltage, on a specific pin on the parallel port. There are

chips,

> like the one on my desk, which are hooked to *each* pin on the

port, and

> will read the ones that have voltage on them, as being a pulse.

That

> pulse tells the chip to move the stepper a single step. Then, the

whole

> sequence is repeated hundreds of thousands of times, with different
> 8-bit binary values each time.
>
> Neat, huh?
>
> There is an "optional" layer that you need to know about. The

parallel

> port is hooked up to your computer's electrical bus, and if one of

the

> motors shorts out, sends a power surge back into the port, or

anything

> like that, it can seriously screw up your computer. Sometimes even

fry

> it. There is a box you can use, which uses light to transmit the
> signals, instead of electricity. It optically isolates the

parallel

> port so that any power surges can't get through...there's no wire
> connecting them! It's a "just in case" safety feature.
>
> Even neater!
>
> Now...you asked about limitations. A standard parallel port has 8

data

> output lines. Creative use of some of the other pins has increased

that

> number by a bit. But, needless to say, the number of data lines

will

> limit the number of motors you can control. In fact, it takes 2

pins to

> control each motor...a pin to trigger a step, and another one to

tell it

> what direction to move...forward or back, also coded as a 1 or a 0.
> Another limitation is the speed at which the parallel port

operates --

> it can only deliver "so" many pulses to the motors, in a given

period of

> time...and that depends on your computer as much as anything else.
>
> -- Chuck Knight