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2-2 Series 90-30/20/Micro Programmable Controllers Reference Manual
–
September 1998 GFK-0467K
2
Section 1: PLC Sweep Summary
The logic program in the Series 90-30, 90-20, and Micro PLCs execute repeatedly until stopped
by a command from the programmer or a command from another device. The sequence of
operations necessary to execute a program one time is called a sweep. In addition to executing the
logic program, the sweep includes obtaining data from input devices, sending data to output
devices, performing internal housekeeping, servicing the programmer, and servicing other
communications.
Series 90-30, 90-20, and Micro PLCs normally operate in STANDARD PROGRAM SWEEP
mode. Other operating modes include STOP WITH I/O DISABLED mode, STOP WITH
I/O ENABLED mode, and CONSTANT SWEEP mode. Each of these modes described in this
chapter is controlled by external events and application configuration settings. The PLC makes
the decision regarding its operating mode at the start of every sweep.
Standard Program Sweep
STANDARD PROGRAM SWEEP mode normally runs under all conditions. The CPU operates by
executing an application program, updating I/O, and performing communications and other tasks.
This occurs in a repetitive cycle called the CPU sweep. There are seven parts to the execution
sequence of the Standard Program Sweep:
1
.
Start-of-sweep housekeeping
2
.
Input scan (read inputs)
3
.
Application program logic solution
4
.
Output scan (update outputs)
5
.
Programmer service
6
.
Non-programmer service
7
.
Diagnostics