About Trending

Trending in the
Logix Designer
 application lets you view data sampled over a time period in a graphical display for a set of up to eight data elements. Data is sampled at a periodic rate that is configurable from 1 millisecond to 30 minutes. The
Logix Designer
 application lets you create a trend, configure the data elements and display attributes, and save it as part of your project file. Trending has these constraints:
  • You can trend data elements of type BOOL, SINT, INT, DINT, and REAL.
  • You are limited to sampling eight unique data elements per trend.
  • You are limited to one trend for 1769-L23 controller projects.
Trend Server provides real-time deterministic trending functionality, and trends tags in Logix controllers,
PLC-5
 processors, and various drives via ControlNet and Ethernet. With Trend Server, you can trend objects with different update rates or with the sample update rate simultaneously.
How Many Tags Can >
Logix Designer
 Trend?
In the
Logix Designer
 application, the controller supports 32 trending objects (one for 1769-L23 controllers). Each trend view you create creates a trending object in the controller, and each trend view allows up to 8 tags to be trended. Therefore, the maximum number of tags the application can handle is 256 tags (8 * 32).
Factors Affecting Trending Results
The number of tags you can trend on different sample periods depends on many factors, including:
  • The speed of your PC.
  • The amount of memory and hard drive space you have.
  • The size of the paging file in your PC (you should use at least the recommended paging file size).
  • The type of controller you are using and the amount of memory it has.
  • The controller's time slice.
  • How busy the controller is.
  • The type of network you are using (for example, Ethernet is the fastest).
  • How many nodes are scheduled on ControlNet, if you are using a ControlNet network.
  • The maximum number of nodes specified for ControlNet and how many actual nodes you have on the network. For example, if the maximum number of nodes is 99 and you only have 1 node on the network, ControlNet operates very slowly because it has to talk to 99 nodes for each scan and wait for 98 nodes to time out.
Trending Example
In this example, we use a 1000 MHz PC, with 512 Mb RAM, and 2.16 Gb hard disk space remaining. The controller is a Logix 5550 controller, with 1 Mb RAM, a time slice set to 20%, and only one task to calculate, which is a sine wave to produce tags for trending. We use a ControlNet network, with only 3 nodes on the network, with a maximum of 15 nodes allowed. According to these specifications, you end up with the following results:
Sample Period
Number of Tags
If more data is trended:
1 ms
1 (20% slice time) or
4 (90% slice time)
Missing data error is reported – controller or ControlNet limit is reached.
PC is very slow. Opening any other
Logix Designer
 software
Logix Designer
 application view, or running any other application, causes the trend graph to display erratically – PC limit is reached.
Tip:
 The missing data is reported for even 1 tag, however, the graph looks okay. The application only reports missing data once, no matter how many data samples are missing.
2 ms
8 (1 trend view, 40% time slice)
Missing data error is reported – controller or ControlNet limit is reached.
PC is very slow. Opening any other
Logix Designer
 application view, or running any other application, causes the trend graph to display erratically – PC limit is reached.
10 ms
20 (3 trend views)
PC is very slow. Opening any other
Logix Designer
 application view, or running any other application, causes the trend graph to display erratically – PC limit is reached.
20 ms
128 (16 trend views)
The Run button is supposed to be disabled after a trend is running. However, the Run button stays on because the PC does not have the CPU to do it – PC limit is reached.
30 ms
176 (22 trend views)
The Run button is supposed to be disabled after a trend is running. However, the Run button stays on because the PC does not have the CPU to do it – PC limit is reached.
50 ms and up
256 (32 trend views)
No more tags can be trended. The controller does not accept any trend request after this point.
Note that each trend view displayed in the
Logix Designer
 application creates a Trending Object, which is actually a task, in the controller. More trend views create more trending objects in the controller. Trending Objects in the controller consume resources, CPU time, and cause heavy scheduling work that considerably slows trending. Therefore, it is much more efficient to pack all trend tags into as few trend views as possible.
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