Rockwell Automation 8520-9API 9/Series OCI API Developer's Guide - Manual (Page 59)

4–11 OCI Data Server Data Items
Publication 8520-6.6 - August 1999
The OCI WatchList
Once your DDE application makes a DDE request to the OCI data
server, the data item is added to a watchlist maintained by the CNC.
New items are added and old items are removed from the watch list
as requested by the OCI data server. Once the CNC has a watch list
it monitors the specified items in the list for changes. When a
change occurs to a piece of data, the processor (after waiting a
configurable number of coarse iterations) sends the OCI data server
the new information.
If multiple OCI data servers are running on the network, there can be
multiple watch lists on any one control. The 9/Series control is
capable of maintaining multiple watch lists (up to 4 different OCI
stations) and will return data as needed to the appropriate OCI
station. The 9/PC can maintain watchlists for only one data server.
Since the watchlist is maintained on the control’s processor instead
of on the workstation (as is common with more typical DDE
applications) the OCI shows improved network performance over
similar competition by not requiring continuous polling of data from
the workstation. It also prevents continuous transmission of
automatic data on the network as data is only transferred to the OCI
server when an item on the watchlist changes. This is the reason no
polling rate configuration is necessary for the OCI installation.
As more items are added to the watch list, more processing time is
required from the processor. We recommend keeping the watchlist
as small as possible to prevent performance degradation and out of
room on watchlist errors. Though most OCI applications make a
negligible change in performance you should not unnecessarily load
the system up with requests for data until that data item is actually
needed. The maximum combined number of data items that any
processor can maintain on all watch lists is based on what data items
are in the watch list as well as remaining control memory.
Important: If your application uses RSJunctionBox (as used by the
Basic Display Set) you must include an item destroy
command in your logic. Item destroy is used to prevent
the WatchList from filling by maintaining links to data
items that are no longer needed. Refer to page 1–8 for
details on item destroy.
Important: Refer to your AMP reference manual for details on
determining and configuring the OCI WatchList Buffer
Size.
Two watch lists are created on the CNC for every connected OCI
station. See the background/foreground section in this chapter for
details. Systems with extremely large watch lists may need to
increase their AMP configured system scan times.