Rockwell Automation 1747-L5xx SLC 500 Modular Hardware Style User Manual - Manual (Page 30)

Publication 1747-UM011G-EN-P - June 2008
30 Selecting Your Hardware Components
Principles of Machine Control
You enter a ladder logic program into the controller by using the
software. The logic program is based on your electrical relay print
diagrams. It contains instructions that direct control of your
application.
With the ladder logic program entered into the controller, placing the
controller in the Run mode initiates an operating cycle. The
controller’s operating cycle consists of a series of operations
performed sequentially and repeatedly, unless altered by your ladder
logic program.
1. Input scan - The time required for the controller to scan and
read all input data; typically accomplished within a few
milliseconds.
2. Program scan - The time required for the processor to execute
the instruction in the program. The program scan time varies
depending on the instruction used and each instruction’s status
during the scan time.
3. Output scan - The time required for the controller to scan and
write all output data; typically accomplished within a few
milliseconds.
4. Service communication - The part of the operating cycle in
which communication takes place with other devices, such as an
HHT or a personal computer.
5. Housekeeping and overhead - The time spent on memory.
➀
➁
➄
➃
➂
Operation
Cycle
Output
Scan
Service
Comms
Overhead
Input
Scan
Program
Scan
IMPORTANT
Subroutine and interrupt instructions within your logic
program may cause deviations in the way the operating
cycle is sequenced.