Datasources and process variables examples

[Mathis-Hu]

Without Alarm Datasource.

Need further discussion before implementing Alarm datasource.

@shroffk

[shroffk]

I think it is great to include examples on how to use the formulas... given our general recommendation to avoid scripts it would be nice to have alternatives highlighted in the default examples

[kasemir]

For local PVs it would be good to mention the syntax as loc://name(initial_value).
Without the initial value, they start out undefined.

I hesitate about "Channel Access is the default protocol".
Yes, we currently have org.phoebus.pv/default=ca, but there are already sites like ESS which try to have as much PVA as possible. Such sites might soon go with org.phoebus.pv/default=pva.

So maybe say:

ca:// selects Channel Access. pva:// selects PV Access.
If you omit the prefix, PVs default to the org.phoebus.pv/default preference setting.

Note that display files should typically not include the ca or pva selector in PV names.
It is a good practice to start all displays with some 'prefix' macro.
For example, a power supply display uses "$(P):Current".
You can then run that display with P=Accelerator:PS47 or P=Test:PS1, and going forward you can then open those same displays with P=ca://Accelerator:PS47 or P=pva://Test:PS1 as you start to support both protocols.