Re: USB or Ethernet machine control

> How about through a T100 ether hub, with 8 or 10 plugins, one per axis?

Probably not, I think. A CNC control running in linear mode (as
opposed to rapid mode) must not only go where it is told, but go there
in a straight (or appropriately curved) line. And that means very
finely coordinating the motion of every axis that is in motion for the
move. In other words, the code handling acceleration and decelration
of the various axis should probably all be running in the same place.

It has been my assumption when we talk about 'smart' peripheral
control boxes that these things run off of drip-fed g-code, possibly
preinterpreted into something easier to digest, but basically without
tight timing constraints. The tight timing is all inside the box,
between the motion planner, servo loops, and output to the drives.

One could consider keeping the motion planner in the computer, and
using usb or ethernet or whatever to send velocity commands to
external step pulse generators or 'digital' servo amps, however then
you have to make guarantees about the maximum latency of the link -
the time between when the program on the computer desires to transmit
a message and when the recipient box actually receives it. Networks
are subject to congestion, although one could build a dedicated one
just for the machine control with its own network card or usb
controllor. However most operating systems run drivers to control
these interfaces in non-realtime code. So again, it becomes very
difficult to do realtime motion control with these types of links
sitting in the middle of the loop.

Which leaves either realtime code on the PC talking to axis drives via
the parallel port or the simple, hardware based rs232 serial ports, OR
non-realtime code on the pc transfering g code files or some digested
version thereof to an external box which has the smarts to do the
reealtime work of making coordinated moves in all the axis.

Or at least that is my understanding of it,

Chris Stratton

PS - one idea that may have some merit is using an old pc as the slave
box. Many of us favor open systems design, and that is a lot harder
to accomplish with anything that has a hardware component unless the
necessary hardware is readily available to everyone. Of course it's
not a hard leap from talking about turning an old pc into a dedicated
motion box to talking about running EMC on said old pc (perhaps
booting off of cdrom) and managing it via an X windows server or
specialized host application running on someone's desktop windows box.

PPS - maybe we should port emc to one of the game consoles that has
been made to run linux, and use that as a standard hardware platform?
Of course finding outputs to actually control the motors becomes
difficult.