Re: [EXTERNAL] phoebus Tank widget: Limits from PV?

["Kasemir, Kay via Tech-talk" <tech-talk at aps.anl.gov>]

Channel Access has a problem with changing runtime limits.

 

For a double-typed channel, a client could historically subscribe to DBR_TIME_DOUBLE to get timestamp, status, value,

or to DBR_CTRL_DOUBLE which sends all the limits, units, precision, …, and the value, but NOT THE TIME STAMP??!

 

So well-formed clients used to read the DBR_CTRL_DOUBLE once, then subscribe to the DBR_TIME_DOUBLE.

That way the client fetched the large and rarely changing CTRL info once, then only got the more efficient time/status/value updates.

That’s great for 99.76% of all use cases, but when you change the limits at runtime, you don’t see that in the client.

 

For a while there’s now a new DBE_PROPERTY modifier for the subscription that allows you do subscribe to the  DBR_CTRL_DOUBLE data but only get updates when limits and units change. That’s the best of all worlds:
Subscribe to property changes on DBR_CTRL_DOUBLE for the 0.24% use case, plus subscribe to the DBR_TIME_DOUBLE for the 99.76% use case.

.. except: Older IOCs and older gateways don’t support DBE_PROPERTY, so you should only enable that if everybody in your system can handle it.

 

See https://control-system-studio.readthedocs.io/en/latest/preference_properties.html#pv-ca for the org.phoebus.pv.ca/dbe_property_supported setting to enable it.

 

See also https://epics.anl.gov/tech-talk/2016/msg01005.php and  https://epics.anl.gov/tech-talk/2021/msg00030.php

 

-Kay

 

 

From: Paul Sichta <psichta at pppl.gov>
Date: Wednesday, July 5, 2023 at 3:35 PM
To: Kasemir, Kay <kasemirk at ornl.gov>
Cc: tech-talk at aps.anl.gov <tech-talk at aps.anl.gov>
Subject: Re: [EXTERNAL] phoebus Tank widget: Limits from PV?

Kay,

Thanks for those tips.  My database file has LOPR/HOPR as -140/160,  but after the IOC is running the LOPR/HOPR are written with new values, e.g. -100/500.

 

When I use the caget command,  the display limits show -100/500.

When I probe the tank widget (PV)  in phoebus it shows, in the metadata section,  display range of -140/160,  and when I look carefully on the tank widget scale it also shows -140/160  range for the case where I checkbox to use the PV Limits - so the PV Limits checkbox is working, but ....

 

How in the heck does phoebus know what the loaded database limits were,  rather than reading the PV's current values?  Is there a cache where it is read by phoebus one time or upon initial connection?  I tried restarting phoebus but it always thinks the display limits are the loaded database's -140/160.

 

Regards,

-ps

 

 

On Wed, Jul 5, 2023 at 2:31 PM Kasemir, Kay <kasemirk at ornl.gov> wrote:

When I try it with this type of record, tank & meter & progress bar all get the 0..10 range from the record:

 

record(calc, "ramp")

{

   field(SCAN, "1 second")

   field(INPA, "ramp")

   field(CALC, "A<10?A+1:0")

   field(HOPR, "10")

   field(LOPR, "0")

}

 

In “probe” you should also see “Display Range: 0.0 .. 10.0”, and from a command line `caget -d CTRL_DOUBLE ramp` should show the same Lo and Hi disp limit.

 

 

 

From: Tech-talk <tech-talk-bounces at aps.anl.gov> on behalf of Paul Sichta via Tech-talk <tech-talk at aps.anl.gov>
Date: Wednesday, July 5, 2023 at 1:51 PM
To: tech-talk at aps.anl.gov <tech-talk at aps.anl.gov>
Subject: [EXTERNAL] phoebus Tank widget: Limits from PV?

Hello, 

I have used  the checkbox to select "Limits from PV'' which I expected would be taken from  LOPR and HOPR of the PV.  That checkbox seems to make no difference;  the tank's range is always the Minimum and Maximum values used for the widget's properties.  I see the same ignoring of PV limits for the meter and progress bar.

 

I'm using channel access for the PV's ,  and version master 2023-06-28 04:51,  but some earlier versions I have do the same thing.

 

Regards,

-ps