Experimental Physics and
Industrial Control System

Benjamin Franksen <[email protected]> · Wed, 15 Oct 2008 21:05:01 +0200

On Mittwoch, 15. Oktober 2008, M.Babai wrote:
> I am trying to develop an application using EPICS CA connection library.
> We have the following setting:
>
> device one: provides PV1
> device two: provides PV2
>
> Now, the intention is to read a number of values from the first device
> and collect them. After collecting of, let say 100, values of PV1 we can
> perform some computations and based on the results we can determine the
> value for PV2 and set that. After this operations we want to go to the
> monitoring phase and continue collecting PV1 values. For reading PV1
> values I am using a program with the same structure like the caMonitor
> application provided by the makeBaseApp.pl:
>
> /Code
> status = ca_context_create(ca_disable_preemptive_callback);
> SEVCHK(status,"ca_context_create()");
>
> status = ca_create_channel(pname,connectionCallback,
>                                 pmynode,20,&pmynode->mychid);
> SEVCHK(status,"ca_create_channel()");
>
> status = ca_add_event(DBR_FLOAT,pmynode->mychid,
> 			    eventCallback, pmynode,&pmynode->myevid);
> SEVCHK(status,"ca_add_event()");
>
> SEVCHK(ca_pend_event(0.0),"ca_pend_event()");
> /Code
>
> I am using ca_pend_event() because I want to collect PV1 values only if
> they change. The frequency of changes of PV1 are not known, thus the
> application needs to wait until there are enough values available.
> If I use the value 0.0 as the argument for ca_pend_event(), then program
> remains calling the eventCallback function (blocking mode), thus all of
> the necessary computations need to be done in the body of eventCallback.
> In the other case, other values for time-out, there is no guaranty that
> we can collect enough PV1-s.

int counter = 0;

while (counter < 100) {
  ca_pend_event(1.0);
}

my_event_callback(...) {
  ...stash the new data value into some buffer...
  counter ++;
}

Or maybe I am not understanding your question?

Cheers
Ben

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System