Re: Analog output device support design

[Ralph Lange <Ralph.Lange@bessy.de>]

 Hi Angus,

On 25.07.2010 20:32, Angus Gratton wrote:
> [...]
> I'm thinking that each record's OUT link can specify what jumper
> settings are applied on the channel, ie OUT, "#C2 S3 @5v BIP" says
> channel #3 is jumpered 5v Bipolar.
>
> The device support can then set default EGUF and EGUL values that will
> automatically work via linconv to convert VAL written in volts to RVAL
> which is the 16-bit integer that goes to hardware.
>
> If the database designer wants to layer a more meaningful value over the
> volts (ie degrees) then they create another database CALC record that
> set the volts in the AO record.

As Mark pointed out, this is generally the way to go.

Note, though, that you don't need the additional CALC record to convert 
from something else. Instead of overwriting, Device Support should *use* 
the EGUL and EGUF values that the db designer created.
That way an ao with "...@5V BIP" and EGUF/EGUL = +/- 5 will convert from 
"output voltage" to the 16-bit integer your card needs, while setting 
EGUF/EGUL to something else will do the linear conversion from that 
"something else" range to your raw value. This is the whole point of 
having EGUF/EGUL/EGU as design fields - the only thing Device Support 
has to do is calculate the actual conversion factors, as only the Device 
Support reads the jumper info from the OUT link, and knows the bit width 
and other particularities of that device.

Please do not write your device support starting from scratch. Copy the 
device support for a device that is similar to yours, understand it, 
refactor to get all the names right, then apply the changes necessary 
for your specific hardware. That way you probably will not only learn a 
lot, but also have a better support library in less time.

Good luck!
Ralph