more RE black boxes

> So your saying a single board computer can do the job? Then in
> principle, you can send G-code to the SBC, which can interpret the code
> and issue control signals to the servos or steppers. So where does the
> proprietary equipment come into play? Thanks.

A single board computer can indeed to this job - in fact that is more
or less how cnc machines traditionally worked - a little (as in
processing power and memory, not necessarily physical size) computer
read g-code off of paper tape or had it drip fed via a serial port.
Then it did the necessary calculations (probably using fixed-point
math) and controlled servo drives. I believe Mariss is looking into
making a modern version of this, only using a much more powerful
single board computer.

Some of the windows controls do most of the work on the pc, but have a
little black box that generates step pulses at a more consistent
spacing and finely variable rate (speed) than is easily accomplished
under windows. I believe the computer simply sends them some sort of
velocity and/or distance command. The g-code interpretation and fancy
math is still in the computer.

FPGAs and [embedded] Microprocessors/Microcontrollors have different
but somewhat overlapping capabilities. (In fact, you can 'build' a
moderate powered microprocessor inside of a large FPGA).
Microprocessors are good for thinkinig things out - how do I
accelerate and decelerate, take corners, do circular interpoloation,
etc. FPGA's are better at doing lots of simple things at once -
counting a bunch of quadrature inputs from multiple encoders, counting
step commands from a computer, running fast servo loops, generating
pulse width modulation or chopping, etc. Many of these things are
much more difficult to do in microcontrollor software where only one
thing can happen at any given time - ie, it is harder to watch
multiple pulse sources without the risk of missing one of them if they
come too quickly. So some of the microcontrollors have built in
peripherals like pulse counters and generators - the problem is that
these are not always well suited to cnc applications - we need things
like counters that can count up AND down, in order to track encoder
motion, wheras the usual ones supplied can only count in one
direction. You can buy dedicated chips like the LS7266, but that 2
channel encoder counter costs nearly as much as a small fpga - and the
more chips on the board the more expensive and special purpose it is.

A box that runs g-code or the like itself is probably best built with
a microprocessor. A device which helps improve the performance of a
pc-based cnc controllor may do better with an fpga, as may some of the
stepper and servo driver modules themselves. My personal preference
would be to have both a processor and an FPGA working together in a
box, for maximum flexibility.

But I'd also like to see the software-only controls continue to offer
as much as possible, since there is quite a lot to be said for a
controller that one can download off the web and wire up through the
parallel port.

Chris Stratton

-------------------------------------------------
Christopher C. Stratton
Engineer, Instrument Maker, and Horn Player
ccs@... 617 628 1062
http://web.mit.edu/~stratton/www/brassbuild.html