Experimental Physics and
Industrial Control System

Eric Norum <[email protected]> · Tue, 28 Aug 2012 08:45:35 -0700

True, but as far as I can tell from his original message  the underlying problem is that he's mixed up how EOFF and EGUL are used in the standard record conversion routine (apparently there's a version of the record reference manual that has this in error).  If he just changes his database/code to set EOFF properly everything should work as he desires.

On Aug 28, 2012, at 8:31 AM, Dirk Zimoch <[email protected]> wrote:

> Hi Bruno,
> 
> Your device support should have a function specialLinconv() or similar, which is function number 6 in the device support structure:
> 
> struct {
>    long      number;
>    DEVSUPFUN report;
>    DEVSUPFUN init;
>    DEVSUPFUN init_record;
>    DEVSUPFUN get_ioint_info;
>    DEVSUPFUN read;
>    DEVSUPFUN special_linconv;
> } myAi =
> {
>    6,
>    NULL,
>    NULL,
>    initRecordAi,
>    getInIntInfoAi,
>    readAi,
>    specialLinconv
> };
> 
> That function may look like this:
> STATIC long specialLinconv(aiRecord *record, int after)
> {
>    epicsUInt32 hwSpan;
>    myPrivate_t *priv = (myPrivate_t *)record->dpvt;
> 
>    if (after) {
>        hwSpan = (priv->type == LCN12BMP || priv->type == LCN12BBP) ?
> 		4095 : 65535;
>        record->eslo = (record->eguf - record->egul) / hwSpan;
>        record->eoff = record->egul;
>    }
>    return 0;
> }
> 
> Call specialLinconv() from initRecordAi(). Additionally, the record will call it automatically when anyone changes EGUL or EGUF.
> 
> In initRecordAi(), malloc a private data structure (I called it myPrivate_t) with a field "type" (and anything else you need to store and use later, for example in readAi). Set type and then store the pointer to the structure in record->dpvt before calling specialLinconv().
> 
> In the record database, set EGUF to the user value that corresponds to the maximum ADC value (4095 or 65535 depending on your ADC type) and EGUL to the value that corresponds to the minimum ACC value (0). For example 10 and -10 (Volts). You may change that later on the fly and specialLinconv will adjust ESLO and EOFF.
> 
> Dirk
> 
> 
> Bruno Seiva Martins wrote:
>> Hi Ralph,
>> Thanks for your advice! I'm still learning EPICS, so I'm using those A/D's just for experimenting. When it comes to the real deal, I'll keep in mind what you said.
>> Best regards,
>> Bruno Seiva Martins
>> On Tue, Aug 28, 2012 at 11:05 AM, Ralph Lange <[email protected] <mailto:[email protected]>> wrote:
>>    On Tue Aug 28 2012 15:28:34 GMT+0200 (CEST), Bruno Seiva Martins
>>    <[email protected]> <mailto:[email protected]> wrote:
>>>    Rectifying:
>>> 
>>>       epicsFloat64 div;
>>> 
>>>       pao->eguf = 10.0;
>>>       pao->egul = (type == LCN12BMP || type == LCN16BMP) ? 0.0 : -10.0;
>>>       div = (type == LCN12BMP || type == LCN12BBP) ? 4095.0 : 65535.0;
>>>       pao->hopr = pao->eguf;        pao->lopr = pao->egul;        pao->eslo = ((pao->eguf) - (pao->egul)) / div;
>>> 
>>> 
>>>    Bruno Seiva Martins
>>    Another comment:
>>    The linear conversion mechanism is intended to convert the digitized
>>    value into the final engineering units.
>>    You might want to use your A/D converter for signals that correspond
>>    to positions [mm], pressures [mbar], or temperatures [C].
>>    In those cases, the database designer will set EGU, EGUL and EGUF to
>>    e.g. "mm", -5 and 25, meaning that -10V are equivalent to -5mm, and
>>    +10V are equivalent to +25mm. To limit the range that is shown in
>>    the GUI, the designer might also want to set e.g. LOPR to -2 and
>>    HOPR to +2, so that graphs and meters show the "interesting" range,
>>    -2mm to +2mm.
>>    To have this work correctly, setting EGUF, EGUL, HOPR and LOPR in
>>    your device support may actually not be a good idea.
>>    Normally, I would just set ESLO and EOFF.
>>    Cheers,
>>    ~Ralph
> 

-- 
Eric Norum
[email protected]

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System