Re: [Bacnet-developers] COV subscriptions and management

[Steve K. <st...@ka...>]

Hello Salvatore,

>  I'm thinking to develop an application to monitor some values in a BACnet
> device, so I expose my problem an my test environment.

Sounds like a fun project!

>  1) I don't understand, how use the "pid" parameter. I know that pid
> identify a process, but in the COV subscribing context, I would know
>  "who is" and "what does it do" this process that I pass such as parameter
> of  bacnet-stack-0.4.2/demo/ucov/bacucov

pid is just an additional way of identifying that it is your
subscription.  You can just make up this value.

>  2) Why I have to pass the "Value"  parameter?

COV means ChangeOfValue, and the Value is the data that is being communicated.

>  3) With google, I found this:
[snip quote]
> COV subscription is not yet complete.

The current status (in subversion - I don't recall if it is in the
current release) is this:

1 There is the encoding/decoding core file (src/cov.c).
2. There is a demo client application (demo/ucov/) that performs
UnconfirmedCOVNotification requests.
3. There is a demo server application (demo/server/) that handles
SubscribeCOV, but not SubscribeCOVProperty for a demo object (binary
input - see demo/object/bi.c and demo/handler/h_cov.c).  It was tested
with VTS3 because there is not a demo client application for
SubscribeCOV yet.

> 4) I would make a BACnet network  with a
>   SCADA web service and a device named "D_extra" used to receive all (or
> about all) COV events.
>  I would make an interoperability  with BACnet and a defined platform named
> DOMOnet using the "open protocol" of web service to comunicate from DOMOnet
> to BACnet.
>  In this scenario is used the Polling to manage the update fase and this
> introduce  many problem (response time almost linear at number of Objects,
> overload the network with unnecessary messages etc).
>
> Have you any  idea to solve this problem?

Sounds like COV might solve the polling problem.

>  For the (b) comunication, starting from your
> bacnet-stack-0.4.2/demo/ucov/bacucov,  I would use the stack BACnet to
> manage the COV ...
>  This is possible?

Yes, it is possible.  You should use a SubscribeCOV client application
that would fall back to polling by way of ReadProperty if the target
did not support SubscribeCOV.  That application could then store the
data until your main client asked for it.  The SubscribeCOV client
application demo is not written yet.  The method that you should
return the data to your application is probably the BACnet Web
Services:
http://www.bacnet.org/Addenda/Add-2004-135c.pdf
That is not implemented into the BACnet Stack at SourceForge yet. So,
it could be done, but there is some work to do.

Best Regards,

Steve
-- 
http://steve.kargs.net/