Re: BI-Directional PVs

[kuner <kuner@mail.bessy.de>]

I've nearly finished work for an OPC device support, also base3.14 and 
win32, with the same feature:  InOut records. I realised it with the out 
records. In-records are supported  read only with all scan facilities. 
Out-records will be noticed by a Interrupt service routine if any 
changes occure:

The opc-callback handler  - the "hardware access" - reads the hardware 
(the hardware is here a class that implements an OPC client), writes the 
data to VAL/RVAL field and initialises an epics callback:

hwClass::HwCallbackHandler(void)
{ if( recDpvt->isCallback )    // for out-records and in-records with 
SCAN="I/O Intr"
{ readHw();       // read value
    if( recDpvt->isOutRecord )  // out-records are SCAN="passive" so 
scanIoRequest doesn't work
    { recDpvts->noOut=1; //prevent write value back to HW and cause a 
infinite loop
    callbackRequest(&(recDpvt->callback) );
  } else                       // in-records use scanIoRequest
  { scanIoRequest( recDpvt->ioscanpvt );
  }
}
}

The epics callback causes record processing:

static void outRecordCallback(CALLBACK *pcallback)
{ dbCommon *prec;

callbackGetUser(prec, pcallback);
dbProcess(prec);
}

and the write hardware function cares to the noOut-flag

long opcSetScalar(OpcToEpics * pOpc2epics)
{ if(recDpvt->noOut )
{ pOpc2epics->noOut=0;
  return 0;
}
..write hardware and return
}

if there is an attempt to write the hardware from both sides at the same 
time it may be undetermined which value is realy written, but is this 
avoidable if there are no priorities defined?

I'm not shure if this is a good way to solve this problem because i'm 
not very familiar with epics internals, but it works. My code is divided 
in a devSup.c which does the record dependant stuff - DSET, init and io 
routines and mask/shift vfor mbb records - and supports all records that 
have an INP/OUT field and VAL/RVAL. There is a drv.cpp which handles the 
hardware/OPC side - and allows all C++ features :-). It is intended to 
be reused for other "hardware" as OPC only, so you could try to use it - 
if you are interested in.

Gruessle Bernhard