Re: synchronizing the value of a read/write record

["Thomas, Patrick via Tech-talk" <tech-talk@aps.anl.gov>]

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

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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
>