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Subject: Re: Should ai, ao records have RAWH, RAWL?
From: wbrown@csg.lbl.gov (Bill Brown)
To: tech-talk@aps.anl.gov
Date: Wed, 9 Oct 96 08:31:17 PDT
Marty sez:
> Last week during a visit at DESY, Gabor Csuka
> suggested that the ai and ao records should have two additional
> fields RAWH and RAWL. The reason is that often it is possible
> to create common device support for a set of modules that differ
> only in the number of bits and/or polarity.

> I expressed concern that this opens door to an unlimited
> set of configuration fields. After discussion I had to agree
> that these two fields  are generic enough that perhaps
> we should add them. They would not impact any existing support
> but future support could use them.

> [  ...  ]

> Comments anyone?

I'm not sure what this accomplishes.  Hmmmm - mumblemumble - is this
intended to simplify the case of supporting (for example) two ADC boards
which are identical except for the number of bits and the data is right
adjusted rather than left-adjusted?

Off the top of my head this seems like it ought to be hidden in the
device support/driver levels, but I can see where it might be useful.
It could cut down on the number of device types required to support
some of the more flexible boards.

On the other hand, it seems to be at least loosening the lid on a
rather distasteful can of worms wherein the record is starting to know
about the skeletons in the devices' basement that really should remain
hidden.

Perhaps the next step is to provide a polarity field in bi and bo records
and polarity masks in mbbi and mbbo records, just for openers.  Some boards,
like the vmic2534 are available in various inverting/no-inverting flavors.
When I did the vmic2534 driver I hid this detail inside the driver at the
cost of a few cycles.

Not knowing the specifics of the boards that triggered the immediate question
and what the driver can find out about the boards at config time it's a
little hard to comment on how one would go about automagically adjusting
for the precision.

If the precision information is in the record structure,  when is the "proper"
time to look at it and use it?  At Init time?  Every time the record is
processed?  "Hot-swapping" of vme boards IMHO is a comming thing, so perhaps
this question is not as strange as it may seem.  How "dynamic" are these new
fields?  What about "default" values?  There are some dac/adc boards which
can operate in either sign-magnitude bipolar or twos-compliment bipolar modes.

It seems like there would need to be some _very_ carefully thought out rules
for supporting and using this feature, or setting up databases could get
much messier. 

I guess I have to go with Marty's first impression; that this is one path
that we probably shouldn't start wandering down.

My ,02 FrF-worth

-bill


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