Chapter 5
Ladder Program Basics
5–11
The diagram below shows a simplified operating cycle, consisting of the
program scan, discussed in the last section, and the I/O scan.
PROGRAM SCAN
I/O 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
associated on/off state of the bits in the input data file are changed
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. Status 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 output terminals
until the next I/O scan.
Important: The description here does not account for the processor
overhead and communications portions of the operating cycle.
These are discussed in appendix D, Estimating Scan Time.
Operating Cycle (Simplified)
