RS View ME best practices

This is my first project in RS View ME. We need some advice about best practises and performance.
All logic in the PLC done in US imperial units. In our application we provide option to switch units of meassure: metric or US imperials. From performance point of view, what is the best way to handle this:

1) create two identical controls (for example numeric displays)for metric units and imperial units; each has its own HMI tags;each tag scaled accordingly; make these controls visible / invisible depending if option is ON or OFF.

2) create one control; use two HMI tags, one for metric units, one for imperial, and use IF...THEN...ELSE expression in the connection tab of the control.

3) create one control, use one HMI tag, use IF...THEN...ELSE expression and scaling in the tag database, when defining this tag.

Hardware is not available yet. So far everything is tested only on the development PC using RS Studio and RS 5000 emulate, and performance is very similar. During development time I found that managing two objects one on the top of another is not very convinient. If we choose option 2 or 3, development will be much easier. But if later on, when hardware become available, any performance issues will be discovered a lot time will be lost.

Any comments?

Thank you,

PaulB.

Another option might be to use a single HMI tag in imperial units. Create another HMI tag called 'scaleFactor'.

Your display expression could be something like 'myTag / scaleFactor'.

Load the value of scaleFactor according to the user's current unit selection. Let's say you're measuring distance (ie. inches and mm): for imperial units, load 'scaleFactor' with '1', for metric load 'scaleFactor' with 25.4.


-Trevor

Thank you, Tvey.

But what is better to keep expression in the display or in tag database?

PaulB.

Personally, I'd prefer to have a single HMI tag in whatever units the PLC is using - keeps the tag database usable and makes future modifications easier.

Abstracting the 'scaleFactor' to a variable carried on the display makes it obvious what's happening for anyone who needs to edit the display in the future. If your customer suddenly has a burning desire to display everything in rods-per-furlong, all you have to do is add another button to load the 'scaleFactor' tag with whatever the conversion factor is.


-Trevor