NI 9207 has lower Sample Rate When Used in Scan Mode

Updated Jun 29, 2022

Reported In

Hardware

  • CompactRIO Chassis
  • CompactRIO Controller
  • NI-9207
  • sbRIO-9207

Software

  • LabVIEW FPGA Module
  • LabVIEW Real-Time Module

Driver

  • NI CompactRIO
  • NI-DAQmx

Issue Details

I am using an NI 9207 in my CompactRIO. When I programm the module in DAQmx mode or FPGA mode, I am able to reach 500 S/s on a single channel in High-Speed Mode. When using the Scan Mode, I often see the same values returned of the module, which indicates, that the channel is not sampled.

Solution

This behaviour is expected, because of the way, how the cRIO Scan Mode works.

Both in DAQmx mode and FPGA mode, you can configure, how many channels of the 9207 you sample.
If you only configure one channel, this channel can use the full 500 S/s.
If you configure two channels, then the 500 S/s are distributed between the two channels.

When using the NI-9207 in the Scan Mode, all 16 AI channels are sampled, and therefore, a single channel is only sampled at around 31 S/s.
This is independent of how many Scan Mode Variables you read out.

Additional Information

The behaviour of the CompactRIO Scan Mode is true for all C Series multiplexed Analog Input Modules, but most modules' sum sample rate is so high, that even with all channels being sampled, the sample rate per channel is higher than the rate of your read loop. For example, the NI 9201 has 500 kS/s and 8 channels, which results in 25 kS/s per channel.

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