Re: Scaling Database entries

[Dirk Zimoch <dirk.zimoch@psi.ch>]

Hi Andrew

Andrew Johnson wrote:
> Dirk Zimoch wrote:
>
> > if you driver does not set ESLO and EOFF, you can set it in your db. 
> > Calculate manually what to set. I did the same with StreamDevice.
>
>
> ...
>
> > Don't forget to set LINR to "LINEAR", of course.
>
>
> If you want to set ESLO and EOFF yourself, you should set LINR to 
> "SLOPE" rather than "LINEAR" or there's a danger the device support will 
> overwrite your values if someone modifies any of the conversion fields. 
>  StreamDevice might not do so if it doesn't have a special_linconv 
> routine, but then it's not following the full ai/ao standard.  Most 
> analog device supports will overwrite ESLO and EOFF; by relying on non- 
> standard behaviour you're making it harder for someone to replace 
> hardware using a StreamDevice interface with some other device, because 
> they now have to change more than just the DTYP and INP/OUT field of 
> their database records when they move (and they might not realize that).

In 3.13, there was no "SLOPE", and even now I cannot find any 3.14 
Record Reference Manual which would explain the use of "SLOPE". 
StreamDevice does not make a difference between "SLOPE" and "LINEAR" at 
the moment.

> StreamDevice should have some way for the author of a protocol file to 
> specify what the device's raw range actually is for analog records, and 
> it should perform the calculation of ESLO and EOFF from that raw range 
> and the EGUL+EGUF pair.  

I think, I will add that. This can easily be done with two variables in 
the protocol file.

> The other option you might want to support if 
> you don't already would be to control whether any conversions get done 
> at all - many instruments return engineering units anyway, and that's 
> why the analog records allow the device layer to set the VAL field 
> directly and return 2.  This is used a lot for GPIB instrumentation.

That's what I do. If a floating point format is used with ai or ao, the 
device support uses the VAL field. See the StreamDevice documentation 
for each record type.

Dirk

--
Dr. Dirk Zimoch
Swiss Light Source
Computing and Controls
Paul Scherrer Institut
phone +41 56 310 5182
fax   +41 56 310 4413