PV State

[willrogers]

A PV may have a particular value (represented by a VType), but that doesn't include whether the PV is connected or whether it is readonly. These are additional information associated with each PV.

At the moment in the store we have

export interface PvState {
  value: VType;
  connected: boolean;
}

We could add readonly: boolean to this. Looking at the PV.java class in Phoebus, the only other state associated with the PV is the list of subscribers. We currently have this as a separate object in the store.

Another important point is that the vtype may be null (this happens for example when a local PV has not been initialised). That suggests that the pv state should be:

export interface PvState {
  value?: VType;
  connected: boolean;
  readonly: boolean;
}

with a value that may be undefined.

[willrogers]

In CS-Studio, widgets may display null, disconnected and invalid in different ways.

[TimGuiteDiamond]

How does Phoebus tackle this problem then? Or are you saying this IS a state stored with the PV state information in Phoebus?

[willrogers]

Phoebus has a PV object, which has:

• a value (VType) that may be null
• isDiconnected()
• isReadonly()

[TimGuiteDiamond]

Main work here would be adding it in connectionWrapper.tsx and giving a sensible name? Are there any places where this would be expected to cause big problems?

Happy to take this on as I haven't looked at this part of the program too much.

[willrogers]

I think this would be quite simple. I'm not sure what you mean by 'giving a sensible name' though.

[TimGuiteDiamond]

Will this need to be optional? Can a PV which is not connected know whether it is readonly or not? Should this default to True if there is no connection or other information from the PV?

[willrogers]

I added a non-existent PV to Phoebus and it showed up as readonly. It's not clear to me whether that's best but I don't have a good reason to give it different behaviour.

[willrogers]

Same in CS-Studio.

[TimGuiteDiamond]

Yea I'm happy with that. Are we envisioning that this information will eventually be sent from the EPICS interface (ie coniql) ?

[willrogers]

Yes, it comes through coniql. It will be false for local PVs, but we could have sim pv for which it is true.

---

From: TimGuiteDiamond notifications@github.com Sent: 10 October 2019 16:07 To: dls-controls/cs-web-proto cs-web-proto@noreply.github.com Cc: Rogers, Will (DLSLtd,RAL,LSCI) will.rogers@diamond.ac.uk; Assign assign@noreply.github.com Subject: Re: [dls-controls/cs-web-proto] PV State (#53)

Yea I'm happy with that. Are we envisioning that this information will eventually be sent from the EPICS interface (ie coniql) ?

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[TimGuiteDiamond]

Cool.