Re: [foxboro] Clearing Accum block
- From: "Johnson, Alex P (IPS)" <alex.johnson@xxxxxxxxxxxxxxxx>
- To: foxboro@xxxxxxxxxxxxx
- Date: Thu, 20 Oct 2005 11:56:33 -0400
Re: Approach 2 not being FT.
Oops. Editing error on my part. Sorry about that.
I should have said (change in CAPITALS below):
CLEAR
Clear is a Boolean input that sets the accumulator output to zero when
CLEAR makes a zero-to-one transition. The ACCUM block automatically
clamps this input at zero if no connection exists.
Since it works on the rising edge, the reset would occur 1 minute after the
top of the hour. That is, at 10:01 not 10:00 the ACCUM block would be reset.
This approach will reset the accumulator every hour, but not at the top of
the hour.
The three ways that I know to do it at the top of the hour are:
1) Connect it to a CALC output
2) Write an IND block to set it using an external reference
3) Write a script on an AW to reset it.
The problem with the first approach is that one would not be able to manual
reset it from the ACCUM faceplate. The CLEAR parameter would be secured to
the CALC output. Sometimes this is an issue.
The problem with the second approach is that you have to write a sequence
block, but the CLEAR parameter would not be secured and the implementation
IS fault tolerant ESPECIALLY WHEN THE TWO BLOCKS ARE IN THE SAME CP.
The problems with the last approach are:
1) If the AW is down, the reset will not occur. THE APPROACH IS NOT
FAULT-TOLERANT.
2) One might forget about the script during an upgrade and not reschedule
it.
3) One-shots across the Nodebus can eventually get expensive.
Me? I'd use the sequence block unless I knew that a manual reset would not
be needed.
Regards,
Alex Johnson
Invensys Systems, Inc.
10707 Haddington
Houston, TX 77063
+1 713 722 2859 (voice)
+1 713 932 0222 (fax)
+1 713 722 2700 (switchboard)
alex.johnson@xxxxxxxxxxxxxxxx
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