Re: Initialising an ao record from a constant DOL

[Andrew Johnson <anj@aps.anl.gov>]

Benjamin Franksen wrote:
> As i understood it, the device support returns 0 by purpose in order to enable 
> bumpless reboot and what Pete wants is to override this behavior with a 
> constant in DOL. As Bob said this is not possible at the moment. But i think 
> it should be possible to change the ao record support so that if DOL is 
> defined with some constant value then this value takes precedence over any 
> value return from the device support's init_record for bumpless reboot.

If an aoRecord device support's init_record routine is not able to set the 
RVAL field to the actual value read back from the hardware, it *must* 
return with status 2.  If it returns status 0 without setting RVAL the 
record is going to immediately trash the VAL field with whatever it finds 
in the RVAL field (probably 0) back-converted into engineering units, 
which is pretty much going to guarantee a reboot bump.

This is exactly how the bumpless reboot facility that Bob was talking 
about works.  It was meant for any synchronous device support which *is* 
capable of reading the last written ADC value back from the hardware 
registers in raw units, and signals this ability to the record by putting 
that raw data into RVAL and returning 0 from init_record.  If it's a 
device support like GPIB that uses engineering units anyway, it puts that 
readback value into the VAL field and returns a 2.

A device support with is not capable of the hardware readback should 
return a 2 from init_record, to retain the contents of the VAL field as 
set by a constant in the DOL field (which is what Pete was trying to do).

Admittedly the convention used for the value returned there is not 
particularly obvious, but it is documented and does match that of the 
read/write routine with 2 meaning do not convert.

There is no need to make any changes to the ao record to do what Pete 
wants, just fix the device support routine.

- Andrew
--
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