Re: EMC compatable computers

[jmkasunich-at-ra.rockwell.com]

Jon Elson wrote:

> The problem with USB is that it only allows one
> packet every milli-second.  I'm not positive
> that is a real stumbling block, but it certainly
> seems like it could be.  Especially if you wanted
> greater than 1000 servo updates per second.  But,
> even at 1000, the USB timer would be causing timing
> jitter of substantial magnitude against the timer
> EMC uses for the servo cycle.
> It just seems like a real problem.

I couldn't agree more.  I just don't get the infatuation
with things like USB in this application.  USB offers
high average bandwidth.  But it does not offer low
guaranteed worst cast latency.  For CNC, latency is
far more important than bandwidth.

In addition, as far as I have been able to determine,
USB requires some intelligence at both ends of the
link.  It may only take a simple 8 bit PIC, but some-
thing has to read the encoder counters and assemble
the data into a packet.  That means hardware and
firmware development.  Plus, there will always be
some delay for packet assembly/dissassembly, protocol
overhead, and so on.

So what does USB offer that the parallel port doesn't?

John Kasunich