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<== Date ==> <== Thread ==>

Subject: Re: More than 16 enumerations mbbi / mbbo
From: Benjamin Franksen <benjamin.franksen@helmholtz-berlin.de>
To: <tech-talk@aps.anl.gov>
Date: Thu, 26 Jul 2012 15:05:58 +0200
On Thursday, July 26, 2012, Hinko Kocevar wrote:
> On 07/26/2012 02:04 PM, Benjamin Franksen wrote:
> > On Thursday, July 26, 2012, Hinko Kocevar wrote:
> >> While developing writing Asyn device driver support for our products
> >> running on Linux OS, we were faced with the need to support more than
> >> 16 enumeration values that are to be represented using mbbi / mbbo
> >> records.
> >> 
> >> I'm looking for solutions to this problem. We are trying to stay
> >> within EPICS base provided record types as much as possible, but we
> >> have no problems to include additional EPICS extensions to out soft
> >> IOC in order to gain record support. Last resort would be to provide
> >> a new record type.
> > 
> > Writing a new record type with, say, 32 choices based on the existing
> > 16- choices versions wouldn't be hard. The problem is that the result
> > won't be very useful because CA (Channel Access) limits the number of
> > choices for type DBR_XXX_ENUM to 16 which is hard coded in the
> > protocol (see db_access.h). So, for instance, a menu button in an
> > operator panel would not see all 32 choices but only the first 16; and
> > so on.
> > 
> > A possible work-around would be to somehow split the choices into
> > smaller (related) groups, then provide a 'master' mbbi/o for the
> > choice of the group, and separate mbbi/o for chosing inside the
> > groups.
> 
> My version would not even include the 'master' mbbi/mbbo. I was thinking
> about splitting too large enumerations into 16 or less values to fit
> single mbbi/mbbo.
> 
> For example: 20 enums
> - 0 - 15 in mbbi1 record
> - 16 -19 in mbbi2 record

If your choices do not lend themselves naturally to grouping into smaller 
sets of sub-choices, then this approach makes sense. I would, however, use 
only 15 choices, reserving the 16th to indicate that the actual choice 
should be made from the next record in the chain. You could also reserve yet 
another choice to indicate that this record dos not apply, i.e. to indicate 
'undefined' (see below).

You can have arbitraryly many choices if you chain records in this way.

> What will mbbi1 value be if eg. enum 17 should be set?

If you reserve the 16th choice, then "Other" comes to mind, "More..." if you 
think of user interfaces.

> What will mbbi2 value be if eg. enum 4 should be set?

"n/a" or just ""

> Can it be 'undefined'? 

Reserve yet one more choice for this. Or: set the UDF field of a record e.g. 
via db link from another record. If you also have DISS=INVALID you have a 
good idication of an undefined state.

> Or should we leave one enum state reserved in
> this case and set it when other mbbiX is in use..

Yes, see above.

Cheers
Ben

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Replies:
Re: More than 16 enumerations mbbi / mbbo Hinko Kocevar
References:
More than 16 enumerations mbbi / mbbo Hinko Kocevar
Re: More than 16 enumerations mbbi / mbbo Benjamin Franksen
Re: More than 16 enumerations mbbi / mbbo Hinko Kocevar

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