User manual

Rockwell Automation Publication 1769-UM011I-EN-P - February 2013 139
Dynamic Memory Allocation in CompactLogix Controllers Appendix B
Trends
Each trend created in a controller creates a trend object and allocates a buffer for
logging.
DDE/OPC Topics
A DDE/OPC topic uses connections based on these variables:
• Maximum number of messaging connections per PLC controller
configured in RSLinx software
• Number of connections needed to optimize throughput
• Configuration of RSLinx software to use connections for writing to a
ControlLogix processor
Specify Connections per PLC Controller
To specify the maximum messaging connections per PLC controller, perform this
procedure.
1. In RSLinx programming software, from the Communication pull-down
menu, choose Configure CIP Options.
Table 37 - Controller Trends
Item Memory Allocated
Trend Object 80 bytes
Log Buffer 4000 bytes
IMPORTANT
These variables are per path. For example, if you set up two different DDE/OPC
topics, with different paths to the same controller, the variables limit the
connections for each path. Therefore, if you have a limit of 5 connections, it is
possible to have 10 connections, with 5 over each path.