adccommunitymod (AutomationDirect) asked a question.
Created Date: May 03,2013
Created By: skyfox
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Strictly speaking of PLC tags displayed on active screen. (not Tags tied to a Trend) Is C-More polling the PLC for tags that are only on the active screen? Or, does it scan the PLC for all tags that are in a project during a scan cycle? The reason that I ask is that I have a project with over 600 tags already, and the customer wants to add a historical data screen where it will display values for over 80 tags in a single screen. Historical data will be stored in the PLC, and a single historical record has 80 variables (tags). I want to to create a template screen with 80 (text / Numeric) display objects. In the PLC, I have a indexing mechanism implemented so when an increment/decrement button is pushed from the HMI from this template screen, it advances the stored data record by 1 and display the contents of those 80 Tags pertaining to that data record. So my question is when I have the template screen displayed on C-more, is C-more only polling the PLC for those 80 tags? PLC is a Compactlogix if that matters and all connections are ethernet. Thanks
Created Date: May 03,2013
Created by: Do-more PE
C-more scans whatever tags are present on the current screen or on the gray area that is outside of the viewable area on a screen. It also scans an event tags that you might happen to have as well as any line trend objects that are in the project.
Created Date: May 03,2013
Created by: skyfox
Thanks for the feedback.
So is it safe to say that I can have several thousand tags in a project without impacting communications with PLC as long as I limit my tag count on a screen to say 100 or so?
Thanks again. Appreciate your help.
Best Regards
Created Date: May 03,2013
Created by: Do-more PE
It depends on what objects are on the screen and how far apart your PLC memory addresses are.
If your PLC addresses are fairly tightly spaced, say no more than 4 address locations apart C-More will attempt to group the reads and writes if possible to save transmissions. Less transmissions equals faster overall throughput, better performance. (4 address spaces is a guess, it varies based on the PLC driver selected.)
If your PLC addresses are scattered all over the memory space of the PLC, then C-More is forced to do a separate transaction for each address. Very inefficient and it will take a lot longer to retrieve all of the memory addresses. So ergo, slower performance.
Take a look at this whitepaper that shows what factors affect performance.
Created Date: May 03,2013
Created by: skyfox
That was extremly good info. I will definitely keep that in mind for my future projects.
Thanks Again.
Created Date: May 03,2013
Created by: skyfox
Strictly speaking of PLC tags displayed on active screen. (not Tags tied to a Trend)
Is C-More polling the PLC for tags that are only on the active screen?
Or, does it scan the PLC for all tags that are in a project during a scan cycle?
The reason that I ask is that I have a project with over 600 tags already, and the customer wants to add a historical data screen where it will display values for over 80 tags in a single screen. Historical data will be stored in the PLC, and a single historical record has 80 variables (tags).
I want to to create a template screen with 80 (text / Numeric) display objects. In the PLC, I have a indexing mechanism implemented so when an increment/decrement button is pushed from the HMI from this template screen, it advances the stored data record by 1 and display the contents of those 80 Tags pertaining to that data record.
So my question is when I have the template screen displayed on C-more, is C-more only polling the PLC for those 80 tags?
PLC is a Compactlogix if that matters and all connections are ethernet.
Thanks