DAQ Assistant with multichannels causing Simulation Loop slow?

[s3s3]

Hi, another LabView newbie here.

I have in a Real Time Target (NI 9132)  a Control & Simulation Loop with DAQ Assistant block inside, whose signals are fed into a Discrete State Space block. The discrete state space model has 1 second time step. I have set the Simulation Loop parameters so that it executes every 1 second as well (see Fig. A below). *sorry for the big white gap under the figures..

The DAQ assistant acquisition mode is set as "1 Sampe (On Demand)".

However, when I run the VI, the plot seemed to be updated much slower than 1 second rate. To confirm this, I put an "Elapsed Time" block inside the Simulation Loop. The "elapsed time" shows the actual time in seconds while the simulation plot show slower time (see Fig. B below).

I tried to isolate the problem by removing the block one by one. Finally, I found out that this problem was caused by (at least) the DAQ Assistant which acquires multichannels data of NI 9214. When I remove some channels and leave one or two channels, the VI runs at the actual time (see Fig. C below). But when I added more channels reading, it became slower again. 

Here is the snippet of the block diagram (after all other blocks were removed):

What am I doing wrong here? I'm going to use all of NI 9214 channels so how not to have similar problem like this?

I look forward to hearing any relevant comments from the members. Thanks in advance.

Tian

[GerdW]

Hi Tian,

why do you need a Sim loop anyway?

- When it comes to speed you shouldn't use the DAQAssistent. Use basic DAQmx functions…

- Use parallel running loops for each task. Put DAQmx functions in their own loop, running in parallel to your Sim loop…

[s3s3]

HI GerdW,

Thanks for your reply. I used Sim loop because I started to implement my project using some blocks from the Control Design & Simulation Module which required sim loop (at least that's what I remembered).

Thanks for your suggestion, I'll try using the DAQmx functions.

Tian.

[Poolean]

I agree with GerdW's suggestion. Express VIs make prgramming easier, but they also include overhead, which includes: opening and closing references and configuring settings. Performing all of these operations on every loop iteration, may cause your loop rate to be slow.

Mike