Re: USB on Linux

   From: "Gordon R. Hawkinson" <hawkinsong@...>
Subject: USB on Linux

>I have noticed that the newer versoins of Redhat Linux supports USBs.  
>Has anyone tried using the USBs as parallel ports for CNC as the older
>lpt ports are getting harder to find on the newer computers?  I am in the
>process of building a CNC system and I am in the need of using several
>ports. I would like to thank you in advance for any help that someone
>could provide in regards to this question.

Gordon

You can also get usb drivers for many earlier releases of Linux.

I was reading a chapter from Keithley, www.keithly.com about different
connection schemes and got the impression that they thought that usb was
the cat's meow for data acquisition and control. (Keithley, "Data
Acquisition and Control Handbook" 2-9) I'd always thought it sucked.
With as much variability in the time of arrival and double that for the
confirmation and I'd always thought you'd have a pretty unreliable system
or one with big buffers. Nice for plugging in the digital cam when you
wanted to download -- Mandrake 8.1 has a auto icon for this -- but pretty
much worthless when you plugged in a time critical device.

Steve Stallings showed off a MS-Windows based system at NAMES last year
that used some buffering and a usb connection to a driver card on an x-y
table. Looked like it did the job.

I had some discussion with Matt Shaver about this recently and he thinks
that you can consistently get 1 microsecond if you don't try to do
anything else with it. Problem is that you want a bunch of connections
running around the machine or process.

I'd think firewire if I had to do this with serial ports. At least with
it you can predict worse case. And IMHO the boards should be available
for a long time. Or if time is not critical for your tasks some ethernet
stuff can deliver in a predictable manner.

I'm still not ready to give up on the parport.

HTH

Ray