Re: [CAD_CAM_EDM_DRO] First CNC project

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>>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?
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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