Dual scan rate with cRIO

[IanW]

I'm about to start a new project with cRIO and LV 8.20, up to now most of my work has been on SCXI systems. Before I commit to using the hardware I need to know it can work the way I would like.

I've not used FPGA/RT with Labview before, can someone tell me if it's possible to set up a loop reading the 32 channel analogue input module 9205 running at say 1000 scans/s and at the same time a second loop reading the 9211 Thermocouple Input Module at 10 scans/s.

If I can't do that I will have to include both modules in the same fast loop and do some data reduction on the thermocouples. In this case will the DAQ Read throw up an error because the thermocouple can only scan at 14 s/s.

Since this will be my first project using FPGA/Real Time (but several years with Labview), are there any pitfalls I need to watch out for?

Thanking you in advance

Ian

[Christian_L]

Ian,

The short answer is, yes, you can acquire data from each of these modules in different loops at different rates. This will be done on the FPGA in a VI targetted to the FPGA.

In LabVIEW FPGA data acquisition is performed in single point operations. Using the FPGA I/O node you acquire one point of data from each channel of interest. The rate at which you acquire data is determined byt the loop rate in which you place the FPGA I/O node. In your case you would have one loop running at 1 kHz and another at 10 Hz. You do not use any of the traditional DAQ functions that you are familiar with.

If you were to try to run a loop faster than the AI operation allows, the FPGA I/O node would not return control to the rest of the FPGA diagram until the data is acquired and would thus slow down the operation of the loop. You would not see an error in this case and you need to verify the timing of the loop yourself. Developing LV on FPGA is a bit different than Windows in that you are truly designing your own hardware operations and more responsibility is placed on the developer to do their own error checking, etc.

[IanW]

Christian

Thanks for the detailed reply.

It is good news that I can work with the two loops. From your additional comments it looks like I will have to learn quite a few new techniques. Since the funding for the project has been delayed by a couple of months it'll give me time to read up on the topic.

Thanks again

Ian