100Hz/10ms measurements with NI 9214 and scan mode

[GieseUniLu]

Hallo Community,

with my CompactRio and NI 9214 I want to measure temperatures with a sampling Rate of 100 Hz. I am aware that this is not possible using all the 16 Channels.

How do I set it, that the NI 9214 just measure some of the channels - not all - when I use the scan interface mode/engine?

Currently the system records with the 100 Hz measurements pairwise repeated values (see attached jpg). But just 8 Temperatures of the NI 9214 are used at the real time VI. Maybe this problems only occurs, after I had 3 additional Channels used - i am not sure. But I just reopened the old VI and this stays the same.

What do you think?

[crossrulz]

From quickly messing around, it looks like the Scan Engine will always scan all of the channels of the 9214.  You could do the read in an FPGA, but you will have to do the temperature calculations yourself (FPGA interface will only give you the raw voltage).  It might cut out some of the overhead that the Scan Engine performs, but not likely worth it.

[johnsold]

1. I just read in some of the documentation that crossrulz is correct that the Scan Engine always scans all the channels.

2. I do not have the 9214 documentation handy but this is from the 9213:

---

Timing Mode       Conversion Time (Per Channel)     Sample Rate* (All Channels†)

High-resolution           55 ms                                          1 S/s

High-speed                740 μs                                        75 S/s

* If you are using fewer than all channels, the sample rate might be faster.

The maximum sample rate = 1/(Conversion Time X Number of Channels),

or 100 S/s, whichever is smaller. Sampling faster than the maximum

sample rate may result in the degradation of accuracy.

† Including the autozero and cold-junction channels.

---

So it is not clear that you could achieve 100 S/s even with FPGA reads.

Lynn

[johnsold]

I did have the 9214 documentation. It is slightly faster but still has the 100 S/s limitation.

Timing Mode       Conversion Time (Per Channel)     Sample Rate* (All Channels†)

High-resolution              52 ms                                   0.96 S/s

High-speed                   735 μs                                      68 S/s

Lynn

[GieseUniLu]

Thank you for all your answers.

But I am still waiting for someone who knows something more.

At the moment I have not the time to try out the FPGA mode and I will life with 50 1/s.

Maybe this is a concern for future labview versions, to allow the deactivation of channels in the scan mode to speed up the module.

Do Labview employees also reply to these threads?

[johnsold]

NI employees do monitor the Forums but it is not guaranteed that someone will answer. NI employees have blue bars beside their usernames.

Lynn