Experimental Physics and
Industrial Control System

Thomas Birke <[email protected]> · Tue, 16 Mar 2004 12:40:37 +0100

Wang Yanke wrote:
Hi,
 When I tried to archive the vacuum data with
ChannelArchiver, I got some data like:
03/09/2004 15:58:23.254187014   0.000
03/09/2004 15:58:43.254186214   0.000
03/09/2004 15:59:00.000000000   0.000
03/09/2004 15:59:03.254185414   0.000
03/09/2004 15:59:20.000000000   0.000
03/09/2004 15:59:40.000000000   0.000
03/09/2004 16:00:00.000000000   0.000
03/09/2004 16:00:20.000000000   0.000
03/09/2004 16:00:40.000000000   0.000
03/09/2004 16:00:43.254181414   0.000
....
 I have retrievaled it with CGIExport or ArchiveExport many times, but

the results were disappointing.

So, Why can't I get the same proper value(e.g. 1.324e-07) as using

"caget" instead of zero?

Furthermore, why the time interval is not the value i set in the .cfg

file(20 seconds)?

Is there something wrong with the configuration of the ArchiveEngine

or the retrieval tools?

How can I get the data I want?

The reason for that behaviour is, that the Export-functions of the
ChannellArchiver

(every function, that formats floats/doubles into strings) honours the
PREC-field of

the connected PV. It's like

sprintf( "%*f", prec, value )
In some cases, it would be better to use engineering- or
scientific-notation instead of decimal notation - or even ignore the
PREC at all. As an exception, a PREC of 0 is indeed ignored...
We had the same problems with our vacuum values here at BESSY and
started to store the logarithmic values instead.
The good thing is: The archive has all the values with full accuracy, so
once you change the formatting of the output, you'll get your values
just like you expect them to be.

But none of the bundled export tools ignores the PREC-field if it is set
to something other than 0.
The "20 seconds" problem actually isn't a problem.

If a new value arrives via ChannelAccess, it is stored with the accurate
timestamp. The other values are not even stored in the archive (Kay,
correct me if I'm wrong). It's the output-functions again, that produce
these repeated values.
I know this is not really helpful, but it's an attempt to explain what
happens...

Is ther any guideline on whether and how to handle/honour the PREC field
at all?
Thomas

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System