Re: synchronizing the value of a read/write record

[Torsten Bögershausen via Tech-talk <tech-talk@aps.anl.gov>]

Hej again,
as others mentioned, the READBACK functionality should
be what you want.

And there are (more) existing drives to talk to a TwinCAT
system:
https://urldefense.us/v3/__https://github.com/epics-modules/twincat-ads/__;!!G_uCfscf7eWS!caVLbTl9ZEu0xagc8JuokU3hCqOx0HpDOSEb5MerchWocJ4rKHN1zpvKP2I7vXB4rs57HeCpWLCupl3GYPA$ 

That reminds me on my homework, which could give some
inspirataion:
<https://urldefense.us/v3/__https://github.com/epics-modules/twincat-ads/pull/21/commits/51b38f43962d97e30442d26b606db6624a783711__;!!G_uCfscf7eWS!caVLbTl9ZEu0xagc8JuokU3hCqOx0HpDOSEb5MerchWocJ4rKHN1zpvKP2I7vXB4rs57HeCpWLCuobcuM4M$ >

Please feel free to ask more questions.
/Torsten



On 2025-08-13 19:31, Thomas, Patrick wrote:
> Hi Torsten,
>
> This is device support for Beckhoff ADS, which is a communications 
> protocol for reading and writing from and to a TwinCAT PLC. So the other 
> source is anything else that changes the value of the variable in the 
> PLC. This would usually be a user setting it through the TwinCAT 
> interface, or the value being requested through EPICS but not 
> successfully being changed in the PLC.
> Thank you,
> Patrick
>
> ------------------------------------------------------------------------
> *From:* Torsten Bögershausen <tboegi@edom.se>
> *Sent:* Wednesday, August 13, 2025 2:31 AM
> *To:* Thomas, Patrick <pthomas@caltech.edu>; tech-talk@aps.anl.gov Talk 
> <tech-talk@aps.anl.gov>
> *Subject:* Re: synchronizing the value of a read/write record
> Hej Patrick,
>
> Before answering with a longer answer, I would like to understand
> what "the other source" is.
>
> Is it
> a)
>     The same device,that you are controlling ?
>     May be a device that has a user interface on its own,
>     display, keyboard, touch screen ?
>     Or are the values changed throug a user program,
>     engineering tool, something in that style ?
> b)
>     The new values come from a different device, and are
>     stored in a different record.
>     May be a controll-loop where device1 says that device2
>     (the one you are talking about) "please use this value"
>
> Hope this questions are understandable.
>
>
> On 2025-08-13 02:41, Thomas, Patrick via Tech-talk wrote:
>  > Hello,
>  >
>  > I'm attempting to write device support to handle the scenario where
>  > changes to the value of a hardware device can be made from both EPICS
>  > and another source. I would like the value of the hardware to be changed
>  > if I write a value to the record through channel access, and also to
>  > continuously monitor the hardware device for changes made from the other
>  > source and update the value of the record to match. One approach I have
>  > considered is to use separate output and input records, where the output
>  > records write values set from channel access to the hardware and the
>  > input records periodically scan the hardware and update their values
>  > accordingly. I am wondering however if there is a way to do this with
>  > the val field of just an output record. Is there any way to synchronize
>  > things so that write requests coming from channel access are always
>  > written to the hardware and not overridden by periodic updates from
>  > reading the hardware?
>  >
>  > Thank you,
>  > Patrick
>  >