Rockwell Automation 9323-S5500D A.I. SERIES MICROLOGIX 1000 AND PLC-500 SOFTW - Manual (Page 118)

Ladder Program Basics
5-13
Operating Cycle (Simplified)
The diagram below shows a simplified operating cycle, consisting of the program scan,
which we discussed above, and the I/O scan.
I/O Scan
Program Scan
In the I/O scan, data associated with external outputs is transferred from the output data
file to the output terminals. (This data was updated during the preceding program
scan.) In addition, input terminals are examined, and the input data file is updated
accordingly.
In the program scan, the updated status of the external input devices is applied to the
user program. The processor executes the entire list of instructions in ascending rung
order. Bits are updated according to logical continuity rules as the program scan moves
from instruction to instruction through successive ladder rungs.
The I/O scan and program scan are separate, independent functions. Thus, any status
changes occurring in external input devices during the program scan are not accounted
for until the next I/O scan. Similarly, data changes associated with external outputs are
not transferred to the outputs until the next I/O scan.
Note The description here does not account for the processor overhead and communications
portions of the operating cycle. These are discussed in Appendices A and B of the
Instruction Set Reference . Also, for time critical I/O operations, special instructions
such as IOM, IIN, and DII can be used to update I/O terminals during the program
scan.