Re: Initial value readback from hardware into output records

[Benjamin Franksen <benjamin.franksen@bessy.de>]

On Sunday 12 December 2004 04:45, Steven.Banks@doi.vic.gov.au wrote:
> Hi Stephanie,
>
> Thank-you for this.  I understand how to do your solutions (1) and (3), but
>
> it is solution number (2) which I am primarily interested in:
> >(2) For records that have readback available and initialization device
>
> support, >the setpoint is set to the readback by device support at the
> proper time in >iocInit.
>
> I have never seen any concrete examples of how to do this in the real world
> and would appreciate some help in understanding the details.  For instance,
> I presume that the readback must be obtained during Device Support's
> Init_Record() routine but I have not yet fully grasped how this happens.

Take a look at one of the available hardware support modules, see 
http://www.aps.anl.gov/epics/modules/index.php. If you find your hardware 
listed, you are done (mostly). Otherwise, you might want to ask on tech-talk 
if anyone has written devise support for your hardware. Otherwise take a 
second look at the above web page and try to find a similar device support 
and modify the code accordingly. Also, read Application Developer's Guide, 
Chapter on Device Support.

> For instance, the IOC Developers Guide talks about Driver Support and I
> presume that Device Support must call an API provided by Driver Support to
> get the readback value.

Not necessarily. For simple devices, especially synchronous ones, device 
support will usually talk directly to the device.

> But where do I obtain these low-level drivers from?  Do I have to write
> them myself?

Yes, unless you find someone else who already did.

> Or are they generally available somewhere?  

See above.

> Or does Device  
> Support just talk directly to the hardware, and if so what does EPICS
> provide to help with this?

EPICS rovides a library to access devices on the VMEbus under VxWorks. See the 
Application Developer's Guide for details.

Ben