how many compact daq chassis can be connected to a single computer?

[Swimmer]

I am interested in connecting 12 to 16 compact DAQ chassis to one computer.  The chassis would be located at each test position in a durability room.  I would daisy chain the chassis together by using powered USB hubs.  After the last USB hub I'll connect to a Gefen Super USB Externder.  The converts USB 2.0 to Ethernet then from Ethernet to USB Again at the computer.

Because this is a durability test I'm only monitoring 4 analog input, 4 TC and one frequency.  These channels will be sampled at 100Hz and read every 0.5 Seconds. 

I'm confident that the computer can handle this many USB devices but I'm not confident that the drivers can.

Does any body have any experience doing something like this?

I'm considering cField Point but there is a cost issue.  It's double the cost.

thanks

Dave

[Neil_S.]

Dave,

The driver shouldn't have any issues handling that many chassis.  I believe the real issue will come in the powered hubs and the USB extender.  We have had multiple customers complain about performance or other issues when using some USB extenders.  Personally, I have worked with a USB-Ethernet extender that I was successfully able to get to work but the performance was very slow.  You application doesn't require a ton of bus bandwidth so it sounds feasible.  I have not heard of others connecting that many cDAQ chassis to a single computer.  I would recommend trying the extender with a single chassis and benchmarking its performance before going ahead with the other chassis.  In addition, I would recommend using higher quality powered USB hubs.  We have seen some issues with lower quality hubs.

These issues arise in many cases since the hubs and extenders often do not meet the complete USB 2.0 specification.  For most simple devices such as drives, printers, and digital cameras this doesn't tend to be a huge issue.  However, the cDAQ takes advantage of many of the more advanced features of the USB specification, and non-compliant hubs and extenders may cause device failure.

I would recommend you speak with your local sales rep about these options and they may be able to get you some equipment to try.  Please let me know if you have any additional questions.

Regards,

Neil S
National Instruments

Message Edited by Neil S. on 06-01-2007 02:48 PM

Message Edited by Neil S. on 06-01-2007 02:50 PM

[Swimmer]

Neil,
Thanks for your reply.
I was told about somebody at NI who had used a USB to ethernet extender.  I'm under the impression that this person ( possibly you ) used one of the $60 extenders that only supports USB 1.0.  Do you know if that's the case?

The USB extender I mentioned supports full USB 2.0 bandwidth.  And I'm definately paying for that.  I'm hoping that will help prevent any bandwidth issues.  I've also identified powered USB hubs rated at the full bandwidth.

I have been in touch with my local rep and he's hoping to get somebody at NI to try out one of these USB 2.0 extenders and bench mark it.  I haven't heard if he has been successfull. 

I'll see what he can get ahold of and do some bench marking and see what it can handle.

Thanks,
Dave

[KyleBryson]

Dave,

I don't know your exact topology, but keep in mind that the USB spec limits the number of daisy chained devices to a depth of 7, including the host controller in the PC and the device.  I also don't know how many levels your USB extender would use (probably 1, maybe 2).  So, you'll need to use fewer than 12-16 hubs if they are all daisy chained sequentially, or build a topology that connects multiple hubs at the same level in some places in your system.

Regards,
Kyle Bryson
National Instruments

[Moe419]

Can you also combine say a cDAQ-9137 w/ controller and control a cDAQ-9174 to expand the slots you have? Basically use the 9137 to control the 9174?

[Swimmer]

Thanks for the idea.

This led me to realize I may have overlooked another option.

Using the Ethernet connected cDAQ chassis like the 9188.

Could I run several CAT5/6 cables back to a switch and run all of these chassis from one PC?

Any recommendations on the switch?

Thanks,

[Tom_W_[DE]]

Absolutely- you can use USB cDAQ chassis as expansion from cDAQ controllers (like cDAQ-913x) running either WES7 or LabVIEW RT, and you can also use ethernet cDAQ chassis from cDAQ controllers running WES7 only.

Keep in mind that if you need the C Series modules in the slots to be synchronized, you'll need to either use an NI 9469 in each chassis and the controller for synchronization (which will allow you to also use multi-device tasks for ease of programming), or otherwise use PFI terminals to share the necessary clocks and triggers between the chassis and controllers.

[cody_a]

Ethernet cDAQ would definitely be a better option than the USB chassis.  You can use one or two hubs to connect anything and don't have to worry about potential issues with USB hubs that don't meet the full USB 2.0 spec.  Ethernet is a little pricier, but would be a much simpler solution.  And if you were already planning on getting expensive, high-quality USB extenders/hubs the price difference is probably negligible.