Re: [CAD_CAM_EDM_DRO] PC based Controllers with USB?

Fred Smith wrote:

> I have not yet seen a USB based PC controller. Anybody know of one?
> What are the performance characteristics of the USB, as opposed to
> parallel or serial port controllers? Is this a practical approach to
> take for machine control?

We hashed this over pretty well, perhaps that was on the emc list rather
than here,
and the consensus was that there is no upper bound on USB latency.
There is a
bunch of housekeeping that can start at any time and take over the bus
for several
seconds, especially when resolving units added or removed with the
hot-plug
capability. Even if you promise not to add or remove devices while a
motion
control system was running, there is no guaranteed latency with USB.
That is
rather bad news! Also, USB is designed around sending very large blocks
of
info with one request, rather than sending many small requests very
frequently.

With the EPP parallel port, there is no such processor/protocol
overhead, no
multiple bus users stepping on each other's toes, etc. The boards I've
been
designing for EPP parallel port motion control allow pretty fast data
transfers,
and it works very well for small requests, like reading 4 axes of
encoder counters
out. This transfers 12 bytes in about 15 uS. Writing out the DAC
values for
servo velocity command takes 8 bytes in about 10 uS. These times are
certainly reasonable for a 1 KHz servo update rate.

Jon