Can I do this without having to create PI tags to hold the limits?
I can suggest a couple of options to accomplish your objective without creating PI tags to hold the limits. The first suggest is "low tech". Just draw lines and label them as overlays on the trend as shown here:
The second approach requires you set the 'zero', and the 'span' point attributes of the target pi tag. In my example the zero is 0, and the span is 1325. You will notice these are the top and bottom values of the grid because I chose 'Database' in the Max and Min settings in the trend configuration dialog as shown here:
The approach I would take is to create a couple of very simple Calculation datasets in ProcessBook, where the expression is simply the hi and lo limit value. Call each dataset with the appropriate title (LOLIMIT, HILIMIT), and add these datasets to your trend. Moving the trend, maximising it etc won't have any negative effect, as the limit values will be part of the trend as 'virtual' tags along with the process value(s) you are interested in.
You can configure limits in AF as attributes. Everything is prepared in AF to use limits this way, you have already predefined sub-attributes called HIHI, HI, ... of every attribute. Use AF2 or Element Relative reference to show limits on your ProcessBook display.
Having all your limit values in one central place may be easier to manage.
Thanks for the ideas everyone. I would definitely recommend Pawel's suggestion as this incorporates functionality from AF that was designed to meet this exact use case.
To add some screenshots, here's where you would configure the limits and what the trend will look like after:
Right click on the attribute in PSE and select "Limits...", then select which of the limits you would like to add.
This is the result in PSE, showing the new subattributes:
And then add all three to the trend so it looks like this (optionally, configure the colors for the limit lines to be what you're looking for):
If you're not using AF then I would say that the dataset idea is the simplest, but hopefully this shows you how easy AF would be to deliver this functionality.
Retrieving data ...