259Channel object and WriteGroup
- Mar 1, 2011
This morning's conference we discussed the Channel object and WriteGroup service in
the context of using them to write to the LO object Lighting_Command property.
I was tasked with researching this.
In reviewing Add-135-2008aa-PPR1-6 it is clear that the Channel object, as currently
crafted, only supports writing to Primitive datatyped properties. So the constructed
datatype needed for Lighting_Command, as currently written, cannot be used
with the proposed Channel object.
Similarly, the WriteGroup service, as currently written, now only supports
primitive data for the written value(s).
I think on reflection that this is a problem that we must reconsider. There are various
ways to handle this. Although the Channel object and WriteGroup could be
reformulated to allow broader datatypes, it's my belief that this will meet
stiff resistance as there is already a consensus that lead to the current proposals.
We could, of course, not attempt to make any change to the LO object which
would mean that Channel object and WriteGroup could only be used to
write to Present_Value. I think this would be a major compromise in the
ability of this suite of objects and service to meet the needs and use cases
for Lighting however.
So I believe that a better direction would be to rethink the concept of the
Lighting_Command to allow it to be served by Channel object and WriteGroup,
as already proposed in 135-2008aa. To that end, the Charstring and Octetstring
datatypes come to mind. We could define a fixed position datatype (like Date and Time)
as an Octetstring and use it for Lighting_Command instead of the new
constructed datatype currently proposed. However, this is awkward and inflexible.
The Charstring seems more naturally suited to this task.
I know this is going to be controversial, but it may be the best compromise.
I raise these issues because now that we are revising the LO object for its
public review, it would be better to introduce this idea now instead of later.
I'd like to add this to our crowded agenda for the 15th telecon.
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