Rockwell Automation 1772-AF4,D17726.5.3 User Manual, AUX FUNCT PROM - Manual (Page 12)

Programming
Chapter 3
3Ć4
Avoiding Excessive AF4 Execution Times
Table 3.B lists execution times for AF4 functions. To avoid excessive AF4
function execution times, an interlock system is designed into the AF4. This
system automatically checks and does the following:
Permits no AF4 function to run longer than 6ms without returning processor
scan to the processor.
During a program scan each true AF4 function rung which can be completed
in a single scan will be completed as it is encountered. However, upon
encountering a true AF4 function rung which requires multiple program
scans to complete, all other true AF4 function rungs will be “locked out”
until sufficient program scans complete the active AF4 function rung.
Once started, it completes an AF4 function prior to starting the next AF4
function encountered in the user program which has a true rung condition.
Limits the number of enabled AF4 functions in a program to 50. You may
include more functions but you must ensure that no more than 50 are enabled
at one time. This requirement only applies where you have programmed a
function that requires more than one scan to complete.
This time listed in Table 3.B includes:
Overhead for AF4 interlock system
One run through the portion of the AF4 specified by the particular function
To obtain the time required from activation of the input that makes the rung
containing the AF4 function true until the correct answer for the function is in
the data table, you must add the following times to the values in Table 3.B:
Input delay time (from specification for specific input)
One program scan time and one I/O scan time multiplies by the number of
scans specified in Table 3.B
Methods for determining these times are presented in Mini-PLC-2/15, Series B,
Programmable Controller Programming and Operations Manual (publication
1772- 804).