Experimental Physics and
Industrial Control System

"Johnson, Andrew N." <[email protected]> · Sat, 19 Jul 2014 04:43:42 +0000

Hi Jamie,

This seems to be a rather unusual case. It's easy to implement this behavior using a calc record to do the equality comparison, but equality is actually a fairly rare comparison to use for floating point values (other than zero). If X is really an integer you could use longin or longout records instead, you just have to set the LOW and HIGH alarm levels to one either side of X and HYST to zero. If X is not an integer and is read from an ADC say, you could calculate the ADC values in engineering units adjacent to X and use them for the LOW and HIGH fields of your ai/ao record. If X really is a double value you could even calculate the next higher and lower double values and use them, but you need an understanding of IEEE floating point to do that properly.

HTH,

- Andrew

-- 
Sent from my iPad

> On Jul 18, 2014, at 19:10, "Jameson Graef Rollins" <[email protected]> wrote:
> 
> Hi, folks.  I've been wondering about this question for a while, and
> have therefore assumed it's impossible, but I thought I would ask here
> in case there's something I don't know about...
> 
> Is it possible to set an EPICS AI or AO record to alarm on being NOT
> EQUAL to a set value?  In other words, I don't want to set a range, I
> want on alarm if and only if .VAL != X.
> 
> As far as I can tell such a thing is not possible, which has always
> seemed odd to me.  If it's only possible to set a range for alarms
> (HIGH/LOW, HIHI/LOLO, etc.) then it's actually impossible to cover the
> case where you want a record to take on only a specific value.
> 
> I guess the problem is that the alarms activate on equality (VAL == LOW,
> VAL == HIGH).  If instead the alarm conditions were:
> 
>  VAL == LOW
>  VAL >= HIGH
> 
> then we could cover the equality case by setting LOW = HIGH = X, in
> which case we would have an alarm if VAL != X.
> 
> Is it truly impossible to achieve this currently?
> 
> jamie.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System