Rockwell Automation 8520-ARM2 9/Series CNC AMP Reference Manual Documentation Set - Manual (Page 664)

Miscellaneous Parameters
Chapter 37
37-24
Guidelines on Setting System Scan Time
The system scan time must be at least 2ms longer than the critical
foreground maximum value that is determined during the worst case
testing mentioned previously. Increasing the setting beyond this value is
recommended if:
typical part program execution testing indicates that this will provide an
improvement
a large background PAL exists with a low background interval setting
any other system background tasks require additional execution time
Examples of Setting the System Scan Time
These examples are for illustration only. The specific values used may not
be appropriate for your application, however, you can use the examples as
a starting point to help you determine the appropriate system scan time for
your application.
Our application for both examples has relatively long program blocks
(with a large number of characters) contained in the worst case part
program. The system timer screen displays the maximum critical
foreground time as 4 ms and the maximum total foreground time as 6 ms.
Example A
If we set the system scan time parameter at 12ms, this would allow 6 ms
for the part program decode. If the worst case part program block takes
this long to decode, this is the optimum setting for the system scan time.
Example B
This example shows how a system scan time that is set too low can slow
your system processing time rather than increase it.
If we lower the system scan time to 10, this would allow only 4 ms for
block decode. Therefore two system scans would be required to decode
one block. It would require 20 ms to decode each block. This is an
inefficient setting of system scan time.