Synchronizing multiple cDAQ-9188 ehternet chassis

[Src]

What level of synchronization is possible for multiple cDAQ-9188 ethernet chassis? We are evaluating the possiblilty of doing high channel count scenarios with multiple chassis to see if this would be an issue. So far, I cannot find any information regarding this. Any information would be greatly appreciated.

Thanks,

Steve

[MarkGrot]

Hi Steve,

What modules do you want to synchronize?  The only synchronization method currently supported is signal based synchronization which involves sharing sample clocks and triggers via PFI lines.  As long as the distance and cabling isn't an issue, this will work for most use-cases.

The one exception is that it is not currently possible to synchronize tasks across chassis that use AI modules with delta-sigma ADCs, such as the 9234 and 9237.  The DAQmx help has a full list of these modules in the C Series Device Groupings section.

[Src]

Mark,

Thanks so much for the information. Most of our modules are delta sigma(9235,9236,9237) and we are also mixing these with 9215's and 9205's so this may be a deal breaker for us. I did some looking around and it appears as though the EtherCAT cRIO solution could be promising. Any thoughts on this solution? Thanks again for the prompt response.

[MarkGrot]

Hi Steve,

Mixing modules within a cDAQ chassis is fine.  That synchronization is possible, if you want to do it manually, and automatic if you include all of the channels in the same task.  The problem is that you want multi-chassis synchronization.

How many modules/chassis are you looking to synchronize?  It's probably not feasible for you, but depending on channel count and distance, if you could restrict all the delta sigma modules to one chassis, you could use that as your master and have slave chassis full of 9205s and 9215s.  I realize that's a pretty harsh restriction.

Unfortunately, I'm not an expert in EtherCAT.  I'll have to defer to someone else on that.

[Yo001]

Guys,

I have two 9188 (ethernet) chassiss connected through a gigabyte switch to a laptop. One chassis contains a mix of 9205, 9236, and 9234, while the second has only 9234(accel).

I am using Signal Express for setup and recording data @ 5kHz, trigerring the recording from one 9205 analog channel, with 1 s pre-trigger recording.

There is a time offset of about 20- 30 ms between chassis, and I need to syncronize them. Which is the best method to do it with Signal express? 

Thanks.

[NateBraska]

Hi Yo001,

Since it is ethernet, you are probably getting close to as good as it will get. If you had them using a shared line, such as a PFI line to trigger from, you will be able to do much better. Also, SignalExpress also has very limited options with synchronizing so if you are able to use LabVIEW, then I would also recommend it.

Regards,

Nathan B

Applications Engineer

National Instruments

[Yo001]

Thank you Nathan.

Processing more data, I learned that the offset is not constant from log to log, it's rather ramdomly.

Since I am using Signal Express, and being two separate units, it won't allow me to put them in the same task, therefore the offset.

I am thinking of swapping an accelerometer module from the second chassis with one analog input module from the first, and share an analog signal (voltage) between chassis. Then I can trigger both chassis from it.

At least I'll have a reference for removing the offset later in processing the data.

I am not sure if I can use the PFI chassis ports for the same purpose, can I?

[NateBraska]

Hi Yo001,

That sounds like a good plan. And from how you describe it, you should be able to use the PFI lines on the chassis to trigger from.

Chapter 4 of the 9188 manual briefly talks about the PFI lines of the chassis.

Regards,

Nathan B

Applications Engineer

National Instruments

[lis@jiang]

Hi Yo001,

The problem with synchronizing DSA cards (your 9236, 9234) is that in order to do so, you need to export a clock.  DSA cards take an oversample clock of 100 MHz, but PFI lines are limited at 1 MHz.  You can synchronize the two chassis if you put all your DSA modules in one chassis and use it as the master device, exporting the clock to the slave chassis.  Your other limitation is with SignalExpress, which doesn't offer enough functionality to effectively synchronize 2 cDAQ chassis.  Switching to LabVIEW would allow you to start data acquisition at the same time, but because if you can't fit all your DSA modules in one chassis, you won't be able to use the same sample clock, and you'll eventually see drift in your signal.

Please let me know if you have any additional questions!

Cheers,

Lisa

[NateBraska]

Hi Yo001,

I just wanted clarify my statement earlier. I missed that you said you were using an analog signal. With SignalExpress it looks like you will have to generate/supply a digital signal externally to send to each chassis' PFI lines. With LabVIEW you can use a module, such as a 9205, to trigger from a analog signal. It can then be used to trigger the rest of the modules and the other chassis with a method like this Developer Zone page:

http://zone.ni.com/devzone/cda/tut/p/id/5376

Let me know if that works for you or if you need any more assistance with this.

Regards,

Nathan B

Applications Engineer

National Instruments