User manual

Publication DNET-UM004A-EN-P - March 2004
11-4 Tune the Performance of a DeviceNet Network
The following diagram show the effect of a change to the poll ratio:
Interscan Delay
The interscan delay determines how long the scanner waits before it
starts another I/O scan. In general, leave it at its default value. Change
it only if you need to tune the performance of your system.
background
poll
background
poll
background
poll
background
poll
background
poll
↓↓↓↓↓
I/O scan
poll ratio = 1
scan scan scan scan scan
background
poll
background
poll
background
poll
↓↓↓
I/O scan
poll ratio = 2
scan scan scan scan scan
IMPORTANT
Keep the (foreground to background poll ratio) x (interscan delay) ≤
75 ms (default expected packet value). Otherwise time-outs may
occur.
For example, if you leave the interscan delay at 10 ms, then keep the
poll ratio ≤ 7.
Parameter: Description: Default setting:
interscan delay • Last segment of the I/O scan
• Follows the last polled request
• Provides time for larger devices and slower responders to return their polled
data
• Provides time for software such as RSLinx and RSNetWorx to access the
network for upload, download, browse, etc.
• Scanner waits the interscan delay before it strobes or polls devices again.
• A shorter interscan delay may improve the update time of strobed or polled data.
• Keep the interscan delay ≥ 5 ms. Otherwise you may have trouble accessing the
network.
• Keep the (foreground to background poll ratio) x (interscan delay) ≤ 75 ms
(default expected packet value). Otherwise time-outs may occur.
10 ms