Re: [CAD_CAM_EDM_DRO] RE: PC based CNC system architecture

Jon,

In a USB system there is one Master that generates the 1 msec. Start Of
Frame (SOF), this is usually the host controller inside the PC. The
attached devices may use this as a timing source. Most USB device
processors generate an interrupt when a SOF is detected. I indeed use this
signal for timing in some of my existing microcode.

John

----- Original Message -----
From: Jon Elson <jmelson@...>
To: <CAD_CAM_EDM_DRO@egroups.com>
Sent: Sunday, December 10, 2000 2:04 AM
Subject: Re: [CAD_CAM_EDM_DRO] RE: PC based CNC system architecture


>
>
> John Beidl wrote:
>
> > I think the use of USB is a reasonable solution. It is true that
> > transactions occur at 1 msec. rates, however, that does not mean it
takes
> > one more millisecond to get a reply from a device.
>
> The problem is that for the servo control loops to be stable, they need
> a very predictable sampling interval. If the USB 1 ms. message dispatch
> clock cn be synchronized to something at the PC, that should be OK.
> If the CNC program can synchronize to a stable and predictable
> 1 ms clock from the USB, that might also work OK. But, if that
> 1 ms is not predictable, and/or can't be synchronized or sync'd to,
> then it is a big problem. Everything will be fine, and then suddenly
> a message gets delayed by a whole extra mS, that would cause a
> sudden jerk in the servo.
>
> Jon
>
>
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