Re: [foxboro] I/O Bad Problem

If the block is indicated BAD I/O on an 4-20ma Analog Input, the input
signal is (most likely) going above 20ma or below 4ma. 
There is a capability to widen the tolerance by adjusting the OSV parameter.
However, you should be aware that there is a practical limit to how large it
may be.

As Jim Kahlden wrote to the list the other day:

"Just thought I would pass on what we found when setting the OSV on AIN
blocks that use an SCI of 3 (4-20ma).  Don't use a number greater than 2.5
for OSV if you want the to get a BADIO alarm if a transmitter fails above
20ma. We  changed the OSV on some of our blocks to 3 and some to 5 to stop
BADIO alarms when transmitters drifted below 4 ma and this worked fine.  The
problem came when we had a transmitter fail on the high side and we had an
OSV of 3 and never got the alarm.  After doing some research we found out
why.  The blocks raw count was at max count of 65535, and the transmitter
was at 22ma.  When you do the math you will find that there are 3200 counts
per milliamp and if you get a value of 22 ma you would expect a BADIO.
However since we had set the OSV to 3 this is 1536 counts and that added to
64000 is 65536 which is one more than the max count that we had so no BADIO
alarm.  This was an indication only feedback of a valve which we did not
have a process alarm assigned, only the BADIO.  We had assumed the valve was
going to 100% open when actually it was a false indication because we did
not get the BADIO alarm. =20

Hope this helps someone else from running into the same problem we did if
you plan to change the default OSV settings of 2."


In general, I'd start by checking the loop to see why the transmitter isn't
operating in the 4-20ma range.

Regards,
 
Alex Johnson
Invensys Systems, Inc.
10900 Equity Drive
Houston, TX 77041
713.329.8472 (voice)
713.329.1700 (fax)
713.329.1600 (switchboard)
alex.johnson@xxxxxxxxxxxxxxxx

-----Original Message-----
From: foxboro-bounce@xxxxxxxxxxxxx [mailto:foxboro-bounce@xxxxxxxxxxxxx] On
Behalf Of prabhakar
Sent: Monday, May 15, 2006 12:49 PM
To: foxboro@xxxxxxxxxxxxx
Cc: brad.s.wilson@xxxxxxxxxxxxxx
Subject: [foxboro] I/O Bad Problem

Dear All,

Can anyone tell me why do we face I/O Bad problem with Analog Inputs more 
often than digital inputs. IS ther any option in Foxboro ICC for filtering 
Cyan or I/O bad in DIgital inputs. Of course there is option for Analog 
inputs

Do we hav to write a calca for avoiding this

rgds
Prabhakar
Instrumentation Engineer
Refinery,
RIL,India 
 
 
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