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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:
In the National Instruments data acquisition card both E-series and M-series 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 shown in figure 1


Figure 1: Interval Scanning 

and Round Robin Scanning shown in figure 2

Figure 2: Round Robin Scanning

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 (usec)
0
0
0
0
1
1
0
2
2
0
3
3
1
0
4000
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 pf 1 msec:

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.

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: 06/17/2009
Document ID: 1DAIG7W3

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