Experimental Physics and
Industrial Control System

"M.Babai" <[email protected]> · Wed, 15 Oct 2008 12:10:24 +0200

Hello,
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.
Any ideas how to solve this?
Any suggestions or examples are very welcome.
Thanks in advanced,
/MB

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System