Allen-Bradley 1769-SDN - Page 49

Publication 1769-UM009B-EN-P - May 2002
Configuring the DeviceNet Network 4-13
Foreground to Background Poll Ratio
Devices set for polled behavior can be polled on every I/O scan
(foreground) or they can be polled less frequently (background).
Setting a device for foreground or background behavior is done when
you configure each device in the scanner’s input section.
Figure 4.1 Foreground/Background Polling Behavior
Module Defaults
Clicking the Module Defaults button sets Interscan Delay and
Foreground to Background Poll Ratio to the following values, which
are:
• Interscan Delay: 10 msec
• Foreground to Background Poll Ratio: 1
Poll Ratio = 1
Poll Ratio = 2
Poll Ratio = 4
F
X
B
1
B
2
B
3
B
4
F
X
B
1
B
2
B
3
B
4
F
X
B
1
B
2
B
3
B
4
F
X
B
1
B
2
B
3
B
4
F
X
B
1
B
2
F
X
B
3
B
4
F
X
B
1
B
2
F
X
B
3
B
4
F
X
B
1
F
X
B
2
F
X
B
3
F
X
B
4
Each waveform shows polling
frequency for the given Foreground to
Background Poll Ratio
Where:
F
X
= x foreground slave devices
B
1
= 1st background slave device
B
2
= 2nd background slave device
B
3
= 3rd background slave device
B
4
= 4th background slave device
Allen-Bradley Drives