adccommunitymod (AutomationDirect) asked a question.
Created Date: September 12,2007
Created By: Steven
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Does anyone have any information or documentation on the Scan Control feature in PLCs? I stumbled upon it in a DL06, via PLC- >Diagnostics- >Scan Time- >Setup in DirectSoft 4. It appears to be a way to set a fixed scan time, or a limit to the scan time. But I'm curious how this works, specifically what happens if a scan is not completed when the limit is hit, and if there's a way to configure the limit from in ladder code. Anyone use this feature? Searching through the forums and the manuals has only told me bits and pieces. Thanks!
Created Date: September 13,2007
Created by: AZRoger
Steven,
I don't have DS 4 up anymore, but my guess is that the Scan Control feature is for permanently changing the Watch Dog Timer. It defaults to 200ms (on the D2 series stuff) but can be made longer if need be. This is probably how you'd do that.
Roger
Created Date: September 13,2007
Created by: KPrice
Steven, yes, some CPUs allow configuring the scan, such as the DL450. You can configure the scan to be Variable, Fixed, or Limit. The DL405 manual explains this, but the basics are:
Variable : scan runs as fast as ladder execution.
Fixed : delay is added to scan to meet requested fixed scan time.
Limit : runs variable scan, but gives watchdog error if scan exceeds specified amount.
If the scan exceeds watchdog time, CPU goes into PROGRAM mode.
As this is setup in the scratch pad, I do not believe you can configure from ladder. Hope this helps.
Created Date: September 17,2007
Created by: Steven
Thanks for the replies guys. KPrice, that does seem to match what I had found experimentally. I think the settings may be modifyable by ladder, there are scan control registersin the memory map.
I'll keep this in mind in case I ever find an app where a fixed scan time is helpful. http://forum1.automationdirect.com/board/smile.gif
Created Date: September 12,2007
Created by: Steven
Does anyone have any information or documentation on the Scan Control feature in PLCs? I stumbled upon it in a DL06, via PLC->Diagnostics->Scan Time->Setup in DirectSoft 4. It appears to be a way to set a fixed scan time, or a limit to the scan time. But I'm curious how this works, specifically what happens if a scan is not completed when the limit is hit, and if there's a way to configure the limit from in ladder code.
Anyone use this feature? Searching through the forums and the manuals has only told me bits and pieces. Thanks!