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Subject: PV for both input AND output
From: Matteo Pardi <[email protected]>
To: [email protected]
Date: Thu, 16 Jan 2014 16:33:54 +0100
I have a question related to the in/out PVs used in an IOC .
Can I create a process variable which will have a callback to read data, and a callback to write data?

Let's make, for example, we have an "ao" PV, which the IOC uses to write the data into the hardware.
The database entry loaded in the IOC will be something like

record( stringout, "pVariable")
{
    field( DESC, "this is an example process variable")
    field( DTYP, "blabla Item")
    field( OUT, "@pVariable");
}

This PV will be handled from the EPICS framework by calling the callback "write_ao", every time a client writes the "1" value in it. So in the "write_ao" function we'll provide the code necessary to write the data into the hardware too. Well, if a client write the value "1" in pVariable, the EPICS internal structure variable is up to date (the one which is the formal parameter with "aoRecord*" type in the callback), and the hardware has received the write command (both show the "1" value). Then we connect a different standalone software (independent to EPICS) to the hardware , and we change the pVariable to "2". This variable is an "ao", so it has only one callback for write, the "write_ao" callback, and there is no way to notify it the pVariable value has been changed from the outside. So the pVariable value in the hardware (and in the standalone software) is "2", but EPICS will show "1", because no read have happened.

On the other side, if we had created an "ai" process variable, which has the "read_ai" callback to read data, we have no "write_ao" callbacks to write data, in fact is forbidden to write an input process variable.

So my question still remains the same: how to use a single process variable, to both read and write, if I have only one callback, and I need to read and write item values to the hardware, not only when I write a new value. Maybe I missed something in the EPICS documentation, but I really can't find anything about this question anywhere. So the only solution that came to my mind was to use two different PVs, one for read and one for write, but it seems not to be the right one.

Thank you in advance,
Matteo

--


Ing. Matteo Pardi
CAEN SpA - R&D Software Division
Via Vetraia, 11 - 55049 Viareggio (LU)- ITALY
Tel. +39 0584 388 398 - Fax +39 0584 388 959
[email protected] - www.caen.it
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Replies:
RE: PV for both input AND output Mark Rivers
Re: PV for both input AND output Andrew Johnson
Re: PV for both input AND output Ralph Lange

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