Simultaneous USB or USB and PCI hardware data acquisition

[patufet_99]

Hello,

Is it possible to synchronize the acquisition of analog signals from several USB cards or from USB and PCI cards?

Our need in synchrounous accuracy is not very high. Our acquisition rates being of about 1kHz per channel, 1 ms delay would be acceptable.

But we would like to keep the synchronicity for long term acquisitions (several days).

While with PCI cards, there is the RTSI bus to use the same reference clock, I don't know if there is any possibility when dealing wiht several USB cards or with USB and PCI cards mixed together.

I'm using Labview 7.1 and DAQmx, DaqPad6015, PCI-6224, and USB-6009 hardware.

Thank you for any help

[Louis Manfredi]

I know for sure that it is very difficult to establish and maintain synchronization between several data acquisition devices which talk to LabView over rs-232

I'm pretty sure the same is true of USB-- Any synchronization message going through the Windows operating system is going to have an unpredictable delay-- usually less than a dozen milliseconds, but occasionally much, much, more. Also, the timebase of the various devices won't be exactly the same, so over time they will drift out of synchronization. Though for short data sets, or for longer sets if a means of checking and resynchronizing were available, you don't need to worry about this timebase drift.

Consider a Real-time operating system. (LabView RT, etc, even linux should be much better than windows) Also, depending on your installation, it may be possible to add time base references to each data acquisition device. I've used the time tick from GPS recievers to measure the synchronization of independently sampled data streams. (One microsecond accuracy, and pretty cheap, though sometimes difficult to integrate into the DAQ device.)

Best Luck, Hope to hear from others that there's an answer to your question that I didn't know about, since it often bothers me too.

Best Regards, Louis

[Irene_he]

Yes, it's possible with usb daq to synchronize multiple usb daqs, at least I know with HYTEK iUSBDAQ - U120816. It has an external trigger line, if you tie all usb daqs trigger lines together, and use streaming mode of data acquisition (U120816 is up to 32ks/s, which means if you use 8channels in each module, each channel can do up to 4ks/s which is enough for your spec, 1khz). If you enable the external triggering option, all U120816 will only start streaming after the trigger lines go high. Since the detecting trigger line high timing is in the hardware, the delay will be less than 1ms.

And we tested in lab that multiple iUSBDAQs can stream data simultaneously. The testing computer is winxp, 2.4GHZ with 256MRAM, the usb 2.0 ports are prefered (root hub or extension hub).

And I think the windows timing for USB devices is very different than for RS232 devices.

For information about iusbdaq, check out below link:

http://www.hytekautomation.com/Products/IUSBDAQ.html

Thank you.

Irene

[peteski]

Hello,

Is it possible to synchronize the acquisition of analog signals from several USB cards or from USB and PCI cards?

Our need in synchrounous accuracy is not very high. Our acquisition rates being of about 1kHz per channel, 1 ms delay would be acceptable.

But we would like to keep the synchronicity for long term acquisitions (several days).

While with PCI cards, there is the RTSI bus to use the same reference clock, I don't know if there is any possibility when dealing wiht several USB cards or with USB and PCI cards mixed together.

I'm using Labview 7.1 and DAQmx, DaqPad6015, PCI-6224, and USB-6009 hardware.

Thank you for any help

I've had to deal with sychronization quite a bit, and usually the best solution is to have all of the systems running off of a common clock. Most of the PCI cards I've delt with can use external clock sources to trigger their A/D conversions, and its possible the DAQPad and USB-6009 have that capability. If they can, then you might be free and clear for RELATIVE sychronicity. ABSOLUTE sychronicity (to the rest of the world) might be a different matter though.

And don't worry if you don't have an external source handy. You could just use a counter timer on the PCI-6224 and feed the output of that into the A/D clock source pins set on the other equipment.

Unfortunately, I don't have the time right now to check whether the DAQPad or USB-6009 can use external timing sources, so hopefully you can look this up in your documentation

Good Luck!

-Pete Liiva