On 10/16/2016 11:47 AM, Chris Albertson wrote:
> On Sun, Oct 16, 2016 at 1:13 AM, Nicklas Karlsson <
> nicklas.karlsso...@gmail.com> wrote:
>>> The master board sends the latch command on the parallel
>>> port bus to other boards on the bus, as well as the
>>> computer. The computer would have the luxury of responding
>>> before the next timer tick, nominally 1 ms.
> Yes, this is how to do it but it requires two processors
> 1) a big machine like a PC (or ARM cortex-A) that runs most of LinuxCNC
> 2) A much smaller real-time processor that actually moves the signals
> between #1 and the machine tools
> You said a "latch" would work. It would but only in the simple case of a
> parallel output port. You need to also worry about other kinds of data,
> maybe even serial data or PWM . Also the data might not be going over
> parallel port. It might be Ethernet or USB. New computers lack parallel
> printer ports.
The Pico Systems PPMC was developed in 2001/2002, and
follows closely what others have done for real time
control. You latch the machine state (encoder position) at
one tick of the timer, the computer computes new velocities,
and the new velocities take effect when the timer generates
the next tick.
The PPMC, Universal Stepper Controller and Universal PWM
Controller all use FPGAs, so they are quite a bit more
real-time than a CPU. Our FPGA takes the place of the
Yes, USB has its own real-time clock, and won't submit to
anybody else's idea of real time. Ethernet can work, but it
is a lot of trouble. You can put a parallel port in any
computer that has a PCI or PCIe slot. We have found that
not all plug in parport cards work, though.
> In a modern factory they have the computers in racks and maybe 100 meters
> of cable going to the machines. So I think yo might want to allow for a
> large physical distance between #1 and #2.
Pretty rare to have CNC controls 100m from the machine.
> I think the above is pretty much how the FPGA in the Mesa cards works. The
> PC sends data and parameters and the card generates waveforms and meters it
> out in real time.
Yes, but we've been doing it before Mesa was in this business.
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