Allen-Bradley PLC-5 User Manual page 287

Using This Chapter
Program Scan
1
Maximizing System Performance
For Information About
Program scan
Calculating throughput
Input and output modules delay
I/O backplane transfer
Remote I/O scan time
Controller time
Example calculation
Performance effects of online operations
Inserting ladder rungs at the 56K-word limit C-13
Using program control instructions
For information about the time that it takes the controller to execute a specific
instruction, see Appendix D.
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
Appendix
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C-5
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Publication 1785-UM012D-EN-P - July 2005
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