Rockwell Automation 1761 MicroLogix 1000 Programmable Controllers - Manual (Page 79)

Programming Overview
4–3
With the 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
program logic.
input
scan
program
scan
output
scan
service
comms
overhead
Operating Cycle
input scan – the time required for the controller to scan and read all input data;
typically accomplished within
µ
seconds.
program scan – the time required for the processor to execute the instructions in the
program. The program scan time varies depending on the instructions used and
each instruction’s status during the scan time.
Note Subroutine and interrupt instructions within your logic program may cause
deviations in the way the operating cycle is sequenced.
output scan – the time required for the controller to scan and write all output data;
typically accomplished within
µ
seconds.
service communications – the part of the operating cycle in which communication
takes place with other devices, such as an HHP or personal computer.
housekeeping and overhead – time spent on memory management and updating
timers and internal registers.
You enter a logic program into the controller using a programming device. The
logic program is based on your electrical relay print diagrams. It contains
instructions that direct control of your application.
Programming