The problem here is if the change delta is too small you don't get any
value. Still, you could set it to be very, very small, but not zero and get
what you want.
My concern is that normally this request comes when the value to be recorded
is used for validation in a plant, e.g., FDA or EPA, and the exact number is
required.
Following Steve's train of thought though gives the following options to
you:
So, I think your options are:
1) Use a variable with a known "ignore me" value and the "good" value
and toggle between them. For example, put the good value on input 0 of a
SWCH and the "ignore me" on input 1 and use a switch to toggle between them.
This is pretty ugly though.
2) Use a CALC block and store your value in one of its outputs. For one
block period between updates, use the appropriate CALC block instruction to
set the ERROR, BAD, or OOS bit. This will cause a change to be generated.
The value will be the same as before, but the historian will record the new
status. If you were to trend this result, you would see small downward
spikes between good values. Average calculations and the rest should be okay
since the Historian weights the calculation based on the time span for each
good value. 0.5s is pretty short assuming the good value hangs around for a
couple of seconds.
3) Use a CALC block to hold the "official" value. This CALC block will
transfer its input parameter to its output parameter when a signal
(typically a Boolean input) indicates the need to do so. To generate a
change in the historian, set the change delta to a very, very small number,
but not zero (0). A zero change delta will result in a new value in the
historian every 0.5 seconds. However, be aware that this approach requires
the value to change by more than the very, very small delta. If the value
stays the same, no value will be recorded with the time of the event. One
probably should also log the event in the Historian so that they can be
detected.
Regards,
Alex Johnson
The Foxboro Company
10707 Haddington
Houston, TX 77043
713.722.2859 (v)
713.722.2700 (sb)
713.932.0222 (f)
[EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
-----Original Message-----
From: Murray, Steve [SMTP:[EMAIL PROTECTED]]
Sent: Wednesday, August 30, 2000 9:37 AM
To: 'Foxboro DCS Mail List'
Subject: RE: Event triggered data collection
Hi David,
You might have the value you want to record copied to a second
block -
like a real or calc ROxx, and then have that second calue stored by
the
historian. That way your second value only changes when your
counter logic
tells it to, and the historian never "sees" the original value
changing.
Stephen Murray
> -----Original Message-----
> From: David Johnson [mailto:[EMAIL PROTECTED]]
> Sent: Wednesday, August 30, 2000 2:15 PM
> To: [EMAIL PROTECTED]
> Subject: Event triggered data collection
>
>
> Hello all,
>
> I was wondering if there is a good way to do event triggered data
> collection in the legacy (Informix) historian. Ideally I
> would like to
> place a value in the historian only when a counter
> increments. Is there a
> way to do this at the historian configuration level, or do I
> need to write
> some C code to take care of it? Any thoughts on this would
> be appreciated.
>
> Regards,
> David
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