Difference Between Interval Scanning and Round Robin Scanning

Publish Date: Feb 05, 2018 | 0 Ratings | 0.00 out of 5 | Print

Overview

This article explains the differences between interval scanning and round robin scanning.

Table of Contents

  1. Introduction
  2. Interval Scanning
  3. Round Robin Scanning
  4. Additional Resources

1. Introduction

Interval Scanning and Round Robin Scanning differ in scan rate, or the amount of time between sampling each channel in the scan list. Every time the sample clock rises, the channels in the scan list are scanned. For cards with Sample-and-Hold, they can scan through the list and retrieve the values that occurred at the time of the sample clock pulse. However, for other cards, there will be a delay which can be minimized or optimized by the scan rate.

For the following example, a PXI 6070 card will be used to scan four channels with a sampling rate of 250 samples per channel per second.

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2. Interval Scanning

Interval Scanning scans the channels as quickly as possible after the sample clock rises. The interchannel delay is a hardware limitation that limits how quickly the channels can be scanned. This is useful because it minimizes the delay for each sample. However, the signals do not have as much time to settle. The 6070E board has a minimum interchannel delay of 1 microsecond.

Figure 1 and Table 1 demonstrate interval scanning with the minimum interchannel delay for the 6070.   

Figure 1:

 

Table 1:

 

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



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3. Round Robin Scanning

Round Robin Scanning uses the maximum interchannel delay by evenly spacing the channels in the time between sample clock pulses. This allows the signals to settles and allows for accurate data. The following equation shows how the maximum interchannel delay was calculated.

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

Figure 2 and Table 2 demonstrate round robin scanning.

Figure 2:

Table 2:

 

Scan Channel Time (usec)
0 0 0
0 1 1000
0 2 2000
0 3 3000
1 0 4000
1 1 5000
1 2 6000
1 3 7000
2 0 8000
2 1 9000
2 2 10000
2 3 11000

 

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4. Additional Resources

 

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