multi usb daq cards for stain gauge calib

[xiao chen]

Hi guys,

I am new to labview and tring to build a daq system use multiple usb cards. My application is to calibrate an array of 36 strain gauges. I know there are cards that have 40 analog inputs (I use single input from amplifier) but budget is tight, boss want to use cheap multiple cards. I already have a ni6009 and need 4 more ni5008 to get a total of 40 inputs (resolution is not a problem). The guy told me that the problem with many of these usb cards is that they can't synchronize. But I think my application need not to synchronize these cards. I only want to read all the ports at "approximately same" time, so  software timing will do. Is it correct? Will a use one vi to read all these cards?

Another thing is that my computer only have 4 usb ports. Can I use a usb hub or should I use an adaptor?

Is there any other solution? Any suggestion will be welcomed.

[Mr.F]

Hi Xiao,

“Synchronization” can mean anything from devices on either side of the world operating on a clock disciplined to GPS, to starting to devices at about (ie: a few milliseconds difference) the same time. The USB 6008 can accept a digital trigger, so you could make it so that all the USB 6008’s start on the same trigger. The lack of synchronizing ability of this device that “the guy” mentioned most likely has to do with this device’s inability to share a clock. Although each device will start at the same time each devices clock with me slightly off from one another and with time this error will accumulate and could become significant. If that doesn’t sound like a problem and starting all the devices at about the same time is fine then your approach seems reasonable.

You absolutely can use one VI to read each of these cards, you would have to create a different task for each but they can all be in one VI.

In answer to your last question, if you do use a hub, a powered USB would be better, however, I would recommend a true PCI or PCIe USB card in lieu of a hub. USB hub quality varies greatly between manufacturers, and unfortunately I don’t have recommendation of a USB hub use.