Re: ASYN parameter value vs VAL field relationship

[Jörn Dreyer via Tech-talk <tech-talk at aps.anl.gov>]

Hi Andre,

The problem you have is that you assume that an output record will be updated when you change the value inside the driver code.
To achive what you want, you need to define an input record that reads back the value from the driver:

record(ai, "$(P)$(R)$(PUMP)MaxPressure_RBV") {
     field(DESC, "set max pressure for process")
     field(DTYP, "asynFloat64")
     field(EGU,  "bar")
     field(PREC, "2")
     field(INP,  "@asyn($(PORT),$(ADDR),$(TIMEOUT))PROC_MAX_PRESSURE")
} 
 And display this in the GUI. You can not use the forward link of the input record to set the correct value on the output record because this would lead to a infinite loop,
where the records call each other recursively.
If you set in the output record the HIGH and LOW limits for the alarm to the maximum and minimum values of the hardware, you could use those to give the user feedback
that  the the value might be clipped by the code.

Regards

Jörn

Am Montag, 22. Juni 2026, 13:36:42 Mitteleuropäische Sommerzeit schrieb André Favoto via Tech-talk:
> Sorry, I accidentally sent the message incomplete...
> 
> I am trying to control the upper limit for a given PV via asyn (same as DRVH does, but since the value is dynamic, I'd rather handle it in the driver instead of DB links).
> I have the following record:
> 
> record(ao, "$(P)$(R)$(PUMP)MaxPressure") {
>     field(DESC, "set max pressure for process")
>     field(DTYP, "asynFloat64")
>     field(EGU,  "bar")
>     field(PREC, "2")
>     field(OUT,  "@asyn($(PORT),$(ADDR),$(TIMEOUT))PROC_MAX_PRESSURE")
> 
>     info(asyn:READBACK, "1")
> }
> And the asyn code that sets the value is
> 
> else if (function == procMaxPressure_) {
>     double maxPressure;
>     status = getDoubleParam(addr, syrMaxPressure_, &maxPressure);
>     // do not allow process pressure to be set above syringe max pressure.
>     if (value > maxPressure) {
>       asynPrint(pasynUser, ASYN_TRACE_ERROR, "Pressure above syringe limit, clipping to %f\n", maxPressure);
>       value = maxPressure;
>     }
>     if (value < 0) {
>       value = 0;
>     }
>   }
> Later on, inside writeFloat64, I do:
> 
>  if (status == asynSuccess && apiErrCode == ERR_NOERR) {
>     setDoubleParam(addr, function, value);
>     callParamCallbacks(addr);
>     asynPrint(pasynUser, ASYN_TRACEIO_DRIVER, "%s:%s: port=%s, param=%s, value=%f, status=%d\n", driverName,
>               functionName, this->portName, paramName, value, (int)status);
>   } else {
> (...)
> 
> What I would expect is that the record only assumes the value that I clipped, i.e., 0 <= value <= maxPressure
> 
> However, what I see is that the asyn parameter PROC_MAX_PRESSURE is indeed clipped, but the PV value assumes whatever value I put. Assuming maxPressure=517, asynReport gives:
> 
> Parameter 10 type=asynFloat64, name=PROC_MAX_PRESSURE, value=517, status=0
> 
> But I successfully wrote "600" to "$(P)$(R)$(PUMP)MaxPressure":
> 
> > dbgf B02-CSLab:SE-Pumps:SP1MaxPressure
> DBF_DOUBLE:         600
> I understand the asyn driver and the database are on different layers, but I would expect is that the PROC_MAX_PRESSURE param and VAL field of the record stayed in sync with the setup above.
> Maybe I misinterpreted how that is supposed to behave?
> 
> I have seen the same behavior in other modules, so I'd like to check if someone has any ideas on how to handle this properly.
> 
> Thanks, and sorry for the previous noise 🙂
> 
> Cheers,
> André Favoto
> 
> 
> ________________________________
> From: Tech-talk <tech-talk-bounces at aps.anl.gov> on behalf of André Favoto via Tech-talk <tech-talk at aps.anl.gov>
> Sent: Monday, June 22, 2026 13:28
> To: Tech Talk <tech-talk at aps.anl.gov>
> Subject: ASYN parameter value vs VAL field relationship
> 
> Hi folks,
> I am trying to control the upper limit for a given PV via asyn (same as DRVH does, but since the value is dynamic, I'd rather handle it in the driver instead of DB links).
> I have the following record:
> 
> record(ao, "$(P)$(R)$(PUMP)MaxPressure") {
>     field(DESC, "set max pressure for process")
>     field(DTYP, "asynFloat64")
>     field(EGU,  "bar")
>     field(PREC, "2")
>     field(OUT,  "@asyn($(PORT),$(ADDR),$(TIMEOUT))PROC_MAX_PRESSURE")
> 
>     info(asyn:READBACK, "1")
> }
> 
> 

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