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What Is the Difference Between Interval Scanning and Round Robin Scanning?

Hardware: Multifunction DAQ (MIO)

Problem: What is the difference between Interval Scanning and Round Robin Scanning?

Solution: This is best explained with an example. Say that you have an NI 6070E board (a National Instruments E Series data acquisition board), and you want to sample 4 analog input channels (channels 0, 1, 2, and 3) at a sampling rate of 250 samples per channel per second. You can use a scan rate of 250 scans per second, where a scan is defined as acquiring one sample from each of the four channels, which means that one scan will occur every 4 milliseconds (1 second/250 scans = 4 milliseconds between scans).

Unless you have a board that performs simultaneous sample-and-hold, there will be a delay between taking a sample from one channel and then taking a sample from the next channel. This delay is known as the interchannel delay, and there two ways to do a scan, which will influence the size of the interchannel delay: Interval Scanning and Round Robin Scanning (see the attached image, scanning.gif).

Interval Scanning uses the minimum interchannel delay for your hardware (by default, but you can change to a longer delay), so that you sample from each channel as fast as possible during a scan. For this example, we are using an NI 6070E board, which has a minimum interchannel delay of 1 microsecond. So if you start at time t = 0 seconds, then your first three scans will look like this:

Scan	Channel	  Time (microseconds)
 0	   0		    0
 0	   1		    1
 0	   2		    2
 0	   3		    3
	
 1	   0	          4000     (4 milliseconds = 4000 microseconds)
 1	   1	          4001
 1	   2	          4002
 1	   3	          4003
	
 2	   0	          8000
 2	   1	          8001
 2	   2	          8002
 2	   3	          8003

On the other hand, Round Robin Scanning divides the time between scans into equal sections, so that you will have an interchannel delay of 1 millisecond:

1 second / (250 samples/channel * 4 channels) = 1 millisecond between samples

or

(4 milliseconds/scan) / (1 sample/channel * 4 channels/scan) = 1 millisecond between samples

Round Robin Scanning will scan the channels sequentially, taking one sample every millisecond.


Note: You can set the interchannel delay with the AI Config.vi.

Related Links: KnowledgeBase 1HQ8JJ91: How Do I Perform Interval Scanning with Multiple Scans per Interval Instead of One (at High Speed)?

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Report Date: 09/11/1998
Last Updated: 01/16/2003
Document ID: 1DAIG7W3

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