Quote:
Originally Posted by jhersh
It is true that there are many USB data acquisition systems out there, but there is a reason they are not used for control. The bus latency and non-determinism are detrimental to control systems above around 250Hz. The USB only tries to send data to or from the device every 1ms (125us for high speed) or less, depending on bus utilization. Fundamentally, that is why they are called data acquisitions systems and the cRIO is called a programmable automation controller.
You'll notice that even for the cSeries IO modules, we have different chassis to use them in, such as the NI cDAQ-9172 chassis. This chassis is USB and can interact with the same modules that come in the kit. This is for reading lots of data very fast or writing lots of data very fast. It is not made for the read-compute-write single point data path that is needed for control systems.
I hope this clarifies the reason why USB is not used for control. If you have more questions, I'd be glad to attempt to answer them.
Cheers!
-Joe |
First off, Joe, I want to thank you for this discussion thread. The information here will be invaluble to students that do not understand bus architecture / limitations (something I know a bit about
)
Now on the the method behind my madness
:
Our old controller gave us a 40ms slow loop (where we could write PWM outputs). Given that, the capabilities of USB 2.0 (even with latency) would be a dream. I wasn't suggesting USB for output control (even though, in some cases it's possible) but instead for data input from cameras and other sensors.
Now my guess is that the cRio (I havent seen a datasheet on the base unit yet) uses either CAN, PXI or a PCI express bus (My guess is CAN bus). The FIRST demonstration (not really complete
) showed a single ethernet camera at 640x320, 15fps, full color. This (to me) didn't seem to tax either the ethernet port nor the bus/processor but is stated as the capabilities of the cRio. This is why I was looking for USB connectivity and webcams.
So I guess my question is ... is the (
FIRST configuration) cRio capable (framgrab and IR) of more than than 1 camera at this framerate? higher framerates at lower resolution? higher framerates at lower color? Multiple cameras (at all)?