Re: PC based CNC system architecture

Matt Shaver <mshaver@e...> wrote:

> A Servo-to-Go card usually has 8 channels of D/A, 8 channels of
> quadrature input for encoders, and 32 bits of digital I/O (it also

has 8

> multiplexed channels of 8 bit A/D, and an interval timer that can
> generate interrupts to the PC, but I've never used either of these
> features). The system Jon is working on is modular and consists of

an 8

> slot motherboard with a DB-25 connector near one end which connects

to

> the PC through a cable to the parallel port. There are (will be) a
> number of different plug in cards that each perform a different
> function. The list is (so far):
>
> 4 channel 16 bit D/A
>
> 4 channel quadrature decoder
>
> general purpose digital I/O (probably 24 bits, Opto 22 compatible,

not

> invented yet)
>
> machine tool I/O board with estop control, home & limit inputs,

spindle

> control relays (to pick the coils of larger contactors), etc. (not
> invented yet either)
>
> 4 channel stepper motor control (possibly the same board as the
> quadrature decoder board with different pld programming)
>
> Using a combination of up to eight of these boards you could

configure a

> system to control just about any machine tool. Using the example of

the

> Servo-to-Go card you would need 2 D/A cards (4 channels each for a

total

> of 8), 2 quadrature decoder cards (4 channels each for a total of

8),

> and a digital I/O card. Obviously, if you had only 3 or 4 axes to
> control, you could use 3 cards instead of 5.
>
> With respect to stepper motor control, if Jon is successful in

putting

> all the logic necessary into 1 card, then you'd have a motherboard

and

> 1 stepper card (up to 4 axis) (this ignores the digital I/O issue

all

> together).
>

Wally K. replys

Thanks Matt, You are starting to clear things up for me.

Could you tell me what the motherboard consists of. Is it a passive
backplane or does it have smarts on it.

On the proposed 4 axis stepper card you are working on. Can you put
more than 1 card in the motherbord.

Also on the proposed stepper card why does this ignore the digital
I/O issue all together.

> > Wally K. wrote.
> > You seem to have a billion dollar card cage in here someware with
> > million dollar connectors. What is wrong with simple cheap header
> > connectors.


> Matt Replys:
> 1. Unless they are equipped with a latching tab they pull apart

rather

> easily, and you can only imagine what happens if you accidentally
> disconnect the encoder cable while the axis drives are enabled...
>

Wally K. replys:

First, header connectors do not pull apart that easily. But it would
depend on board size. Second, how in the world would anything unplug
inside of a control cabinet.

> Matt wrote:
> 2. Most connectors of this type made by Molex, Amp, Panduit, etc.
> require special tooling to apply the plug end , usually a special
> crimper for the contacts. The connector I'm interested in using

combines

> the ease of screw terminal termination with the ability to unplug

from

> the mating connector on the PCB.

Wally K. replys

There is no reason to put connectors that unplug on the axis cards.
Once the cards are in place why would you want to unwire them.

Connectors that are external to the box can be of use but not inside.

> > Wally K.
> > And are you physically building a mechanical card cage.


> Matt Replys:
> Yes, it's actually cheaper than buying a pre made one.
>

Wally K. replys

My point is, depending on card size and layout you do not need
expensive backplane connectors and removable external connectors or a
card cage.

By the way. Is your layout etched in stone and are you getting
boards made yet.


Wally K.

P.S.

I would like to say with the cost and miscellaneous construction
issues aside that it sounds like you are making a very nice control.