Re: Homebrew STG card - USB

[Jon Elson <jmelson-at-artsci.wustl.edu>]

Doug Fortune wrote:

> "Matt Shaver" <mshaver-at-erols.com>  wrote  Sun, 27 Feb 2000
>
> >I'm all for starting with the ISA bus as PCI seems tough to do ....
> >We could probably make a parallel port, serial, or USB to ISA adapter ...
>
> If you wanted to consider PC ->USB->breakout box with I/O's
> then the dirty work has already been done, and almost dirt cheap at
>
> http://www.ActiveWireInc.com/
>
> "ActiveWire-USB with programmable I/O pins that can interface to
>   anything.  Control the pins from your browser by HTML/ javascript or
>   VBasic or C.  Win95/98/2000, Linux, FreeBSD, LabView drivers available
>   NOW! Macintosh next...  "

Great!  Thanks for finding this!  It is very a good start for what we are
looking
at!

One thing that needs to be addressed is worst-case latency on the USB, and
using the USB drivers.  Can this really handle real-time work?  The DAC,
PWM output, or whatever drives the servos, and whatever reads the encoders
needs to be serviced at predictable intervals.  many servo systems I've seen
use a real-time clock to trigger sampling all channels of the encoder counters
and simultaneously update the D/A converters feeding the velocity commands
to the servo amps.  If we had this hardware external, out on the USB gizmo,
then it would send data to the real-time routines of EMC, which would only
have to serve the update before it was needed for the next cycle.

The prices are right, too!

Jon