Experimental Physics and
Industrial Control System

Donny Domagoj Cosic via Tech-talk <tech-talk at aps.anl.gov> · Tue, 30 Mar 2021 08:54:32 +0200

Thank you to everyone for your help and the detailed explanation of the

benefits and merits of each approach. I really appreciate it and it gave

me a deeper understanding on how records are processed and what to keep

in mind when writing new databases.

Regards,
Donny

---
Donny Domagoj Cosic
 Institut Ruđer Bošković, Bijenička cesta 54, 10000 Zagreb, Croatia

Dana 29.03.2021 18:42, Johnson, Andrew N. je napisao(la):

On Mar 29, 2021, at 7:41 AM, Pietryla, Tony wrote:

You could change your calc record to this:

record(calc, "$(KEIT):CalcCurrent")
 {
    field(SCAN, "1 second")
    field(INPA, "$(KEIT):Charge PP")
    field(CALC, "LA-A")
}

Unfortunately the CALC expression doesn’t have access to the fields LA
through LL, although they can be read by other records through DB
links or via Channel Access.

On 3/29/21, 7:33 AM, Donny Domagoj Cosic wrote:

 However, when I corrected this error as you suggested, the result

was

 still always 0. Changing the order of NPP before PP should have

solved

   this problem but it didn't.

Marty’s suggestion has been used as the standard “rate of change”
database in EPICS training courses for well over 20 years, but it
doesn’t work if the record which is the source of the data (your
"$(KEIT):Charge” record) is asynchronous, which yours is since it uses
Streams. When the PP link processes that record Streams sends the
request for data to your Keithley 6514 device, but the device support
is not allowed to wait for a response because that would stop the IOC
until the reply comes back. What happens instead is that after the I/O
operation has been started the PP link just returns whatever value was
in the ai.VAL field at the time and the calculation uses that.

   Using Marks suggestion of setting B to A after the calculation
   ("B-A;B:=A") works as expected and yields the correct result. This

 solves my problem, but I still dont understand why Marty's

suggestion

   didn't work.

Mark’s approach still doesn’t solve the problem that the PP link to
your ai record is always returning the previous reading. That might
not matter in your case, but if it could this database solves that
problem:

record(ai, "$(KEIT):Charge") {
  field(DESC, "Measure Charge")
  field(DTYP, “stream")          # Typo fixed, this was "steam"
  field("@devKeithley6514.proto measureCharge() $(PORT)")
  field(SCAN, "1 second")
  field(FLNK, "$(KEIT):CalcCurrent")
}
record(calc, "$(KEIT):CalcCurrent") {
  field(SCAN, "Passive")
  field(INPA, "$(KEIT):Charge NPP")
  field(CALC, "B-A;B:=A")
}

- Andrew

   Dana 29.03.2021 13:48, Smith, Martin via Tech-talk je napisao(la):

Hi Donny,

Because the Charge record is Passive scan and is being processed
(presumably) only by
the calc record what you are saying makes sense. the calc record
processes the charge
record first for link A using a PP link causing it to get a new
value, then it gets that new

value again using link B without processing (NPP) the charge record.

I think what you really want is this:

record(calc, "$(KEIT):CalcCurrent")
{
field(SCAN, "1 second")
field(INPA, "$(KEIT):Charge NPP") # Get the current value of Charge

field(INPB, "$(KEIT):Charge PP") # Process charge record then get the
new value
field(CALC, "B-A") # You may need to change this to A-B if you want a
positive value assuming the charge is always increasing
}

You need to get the current value first then process the record to
get the new value.

Marty

-------------------------

FROM: Tech-talk <tech-talk-bounces at aps.anl.gov> on behalf of Donny
Domagoj Cosic via Tech-talk <tech-talk at aps.anl.gov>
SENT: Monday, March 29, 2021 6:35 AM
TO: Tech-talk <tech-talk at aps.anl.gov>
SUBJECT: Rate of change calculation

Hello,

I am trying to calculate the rate of change of a record using a calc
record as follows:

record(ai, "$(KEIT):Charge")
{
field(DESC, "Measure Charge")
field(DTYP, "steam")
field("@devKeithley6514.proto measureCharge() $(PORT)")
field(SCAN, "Passive")
}

record(calc, "$(KEIT):CalcCurrent")
{
field(SCAN, "1 second")
field(INPA, "$(KEIT):Charge PP")
field(INPB, "$(KEIT):Charge NPP")
field(CALC, "B-A")
}

The calc record always returns 0. If I change the calc to B+A, to
check
what is going on, I always get double the charge. It seems like the
PP
and NPP are both returning the same value even though the actual
charge
is changing. Any ideas on why this would be happening or suggestions
would be greatly appreciated.

Thank you in advance,
Donny

--
Donny Domagoj Cosic
Institut Ruđer Bošković, Bijenička cesta 54, 10000 Zagreb,
Croatia

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System