Accessing a remote point’s attribute other than value is a point to point operation that generates extra traffic.
Continuum controllers implement an Import/Export strategy in order to minimize network traffic when accessing the value of a point in a remote controller; this is in some ways similar to BACnet COV.
When the value of a remote point is referenced in a PE program the system takes care of making sure the owner of the point sends the value to the subscriber(s) via the Import/Export system ONLY when the value changes.
Consider the following PE program line…
MyNum1 = RemoteCX\Num1
If a looping PE program sits on the line above no extra traffic is generated as the PE program gets the value of RemoteCX\Num1 from its local Import/Export table, information only goes on the wire when RemoteCX\Num1 changes value.
The only problem here is that the Import/Export system ONLY works for the VALUE attribute, any other attribute requested from a remote object results in a point to point message on the network to obtain the requested attribute value.
Consider the following PE program line in a CX controller we will call LocalCX
NyNum1 = RemoteCX\Num1 size
If a looping PE program sits on the line above a number of point to point messages would go on the wire every time the line executes. If there are many requests for attributes on remote objects a degrading in communications could take place.
One way to avoid degrading communications as explained above is expose the value of the attribute in question by placing it in the value of a numeric object.
In RemoteCX create a numeric point (Num1Size) to hold the size of Num1 and use a PE program to set Num1Size
Then the program in LocalCX will look like this…
NyNum1 = RemoteCX\Num1Size
Since the program is now going after the value attribute of Num1Size the import/export system will be used instead of point to point.