Experimental Physics and
Industrial Control System

Kay-Uwe Kasemir <[email protected]> · Tue, 14 Nov 2006 10:13:05 -0500

Hi David:
In the convert()'s

        precord->eslo = (eguf - egul)/(deviceHigh - deviceLow);
        precord->eoff = (deviceHigh*egul - deviceLow*eguf)/
                        (deviceHigh - deviceLow);

the deviceLow, deviceHigh is indeed fixed.
This handles the case where you write device support for a 0..10V ADC,

that happens to return values from deviceLow=0x0000 for 0V to
deviceHigh=0xFFFF for 10V.
Now 'users' can conveniently set the records' LINR=LINEAR,
with EGUL/EGUF/EGU = 0/10/V to see the voltage, or
with EGUL/EGUF/EGU = 2/5/m in case your 0..10V range
represents a liquid level sensor that maps 0V=2m, 10V=5meters,
and so on.
If the deviceLow/deviceHigh is not fixed,
you have to use the LINR=SLOPE method,
and set the EOFF and ESLO fields of the record yourself.
So then you have full control over the rval-to-val conversion,
but instead of configuring something like
{RAWLOW, RAWHIGH, EGUL, EGUF},
which might be more convenient,
you have to compute the appropriate EOFF and ESLO values.
-Kay

On Nov 14, 2006, at 09:37 , David Dudley wrote:
Two point scaling I understand. What I haven't figured yet is
where I put it in.
Where(how) do I set the deviceHigh and deviceLow variables, or is
that expected in the device driver? And if it is in the device
driver, does that mean that I'm going to have to have a separate
device entry for each device that has a different dynamic range?
Dynamic range calculations are not handled inside EPICS?
I've got a number of different dynamic ranges for the different
devices here, do they all have to be separately scaled inside the
device driver to a standard range, or can EPICS be set to adjust
for the dynamic ranges?
Most of the software (make that *all*) I've used previously had the
ability to have the RAWLOW and RAWHIGH values set, and they they
did the scaling themselves. Seems an omission if EPICS can't do that?
David
"Mark Rivers" <[email protected]> 11/13/06 8:22 PM >>>

Device support must have a "convert" routine that does a
calculation like the following to compute .ESLO and .EOFF
        precord->eslo = (eguf - egul)/(deviceHigh - deviceLow);
        precord->eoff = (deviceHigh*egul - deviceLow*eguf)/
                        (deviceHigh - deviceLow);
Mark

________________________________
From: David Dudley [mailto:[email protected]]
Sent: Mon 11/13/2006 8:07 PM
To: [email protected]
Subject: Scaling Database entries

OK guys,
My modbus driver is starting to work, and now I'm linking up with
some of the PLC's in the plant. Now, I've started working on
building the database for the first site.
Here's my question:
I've got some AI signals that come from one of the PLCs. Raw
values on some are scaled 0-32000, while others are scaled
800-32000, and others have raw values from -32768-32767. I see the
definitions for the EGUL and EGUF values, but how do I set the RAW
input ranges? Also, I've got some flow meter values that have to
be scaled from linear to square root values..
David Dudley
City of Corpus Christi

<David Dudley.vcf>

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System