Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 287)

1 Publication 1785-UM012D-EN-P - July 2005
Appendix
C
Maximizing System Performance
Using This Chapter
For information about the time that it takes the controller to execute a specific
instruction, see Appendix D.
Program Scan Since the program scan is comprised of the logic scan and housekeeping, any
event that impacts the time of one segment affects the program scan.
You can monitor the scan time by using the controller status screen in your
programming software.
If no change in input status occurs and the controller continues to execute the
same ladder logic instructions, the program scan cycle is consistent. In real
systems, however, the program scan cycle fluctuates due to the following
factors:
• false logic executes faster than true logic
• different instructions execute at different rates
• different input states cause different sections of logic to be executed
• interrupt programs affect program scan times
• editing programs while online affects housekeeping times
For Information About Go to Page
Program scan C-1
Calculating throughput C-5
Input and output modules delay C-6
I/O backplane transfer C-6
Remote I/O scan time C-6
Controller time C-11
Example calculation C-12
Performance effects of online operations C-12
Inserting ladder rungs at the 56K-word limit C-13
Using program control instructions C-14