Re: [CAD_CAM_EDM_DRO] USB DRO

Jon,

USB provides two type of user data transfer, Bulk and Isochronous.

Bulk data transfer - integrity is guaranteed, but not bandwidth.

Isochronous data transfer - bandwidth is guaranteed, but not integrity (no hardware retries).

USB data is packaged in frames that flow once per millisecond. The one millisecond Start Of Frame (SOF) is generated by the
host and can be used for timing purposes.

I don't know what the latency is between requesting the data to be sent (in Windows, posting an IRP) until the USB transaction
actually starts, however, I suspect it is less than one frame time. I never attempted to measure it.

The PIC parts that Tim mentioned are slow speed 1.2 mbit devices. The parts made by Cypress (they purchased AnchorChip) are
full speed 12 mbit devices and are 8051 based.

Jon Elson wrote:

> james owens wrote:
>
> > I have been speaking to a programmer this afternoon and he is
> > convinced USB is the way to go in all things computer. There is
> > nothing that cannot be done with USB that can be done with other
> > ports.
> >
> > 127 devices can be daisy-chained through one hub with a further 127
> > hubs being daisy-chained. The possiblilty of having a whole workshop
> > running from one computer which each axis controlled by the host
> > computer via a USB hub. Which equates to 16129 axis control
> > simultaneously from one computer. One hell of a CNC control
> > program!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
>
> Well, that's a programmer for you. Not a computer scientist, or
> engineer! DRO, sure, if you
> use an encoder counter chip, then you don't have to worry about latency,
> and the human
> eye is so slow, the update rate is no problem. But, for CNC control,
> you need absolute
> certainty that messages will be processed within a certain time, and the
> update rate is
> hundreds of times higher. I think 8 axes plus digital control is
> reasonable with respect
> to bandwidth of the USB, but I have no idea about latency.
>
> Jon
>
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