Experimental Physics and
Industrial Control System

Benjamin Franksen <[email protected]> · Thu, 6 Nov 2014 14:32:25 +0100

On Wednesday 05 November 2014 09:48:24 Andrew Johnson wrote:
> On 11/05/2014 05:59 AM, Benjamin Franksen wrote:
> > As it turned out (I did not observe the failure myself, and perhaps
> > I
> > misunderstood my colleague) the client wrote the NaN directly to the
> > A field of the subroutine record with no intervening ao record.
> > Using an ao record which forwards its value via OUT link to the
> > subroutine record's A and setting its IVOA to "Dont' drive outputs"
> > would have prevented the problem. Also, the subroutine record
> > probably *did* become INVALID, but this wasn't checked by
> > subsequent records which monitored the subroutine's VAL field.
>
> Note that just having a NaN in one of the input fields of a sub record
> may not result in the record going INVALID, only the VAL field is
> checked.

Yes. It may make sense to to check the input fields, too, but I am not
certain this is what we want in all cases, so there should be the
possibility to disable this. A sub, calc, or calcout record may not use
all its inputs every time (perhaps depending on the values of other
fields) and such unused inputs should not make the record INVALID.

> > One loop hole remains in the ai record: it checks for NaN only
> > inside
> > the convert procedure. Thus, writing a NaN directly to the ai
> > record's VAL will not cause the record to become INVALID (assuming
> > INP is empty). This is perhaps a somewhat non-idiomatic use of an
> > ai record, but there is nothing to prevent it being used like that,
> > so I guess the check for NaN should be made directly inside
> > process() like for the ao record.
> I'm not disagreeing with you and I haven't looked at the code since
> reading that, but I would point out that the ai record doesn't have
> anything like the IVOA/IVOV fields of the ao record, so the NaN
> detection is probably not as important.

The point is that other records may request the value with a MS-link, so
the INVALID state gets propagated, which in turn may prevent bad things
from happening (if these records have IVOA set appropriately).

Cheers
Ben
--
"Make it so they have to reboot after every typo." ― Scott Adams

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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System