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
2-2 | Series 90-30/20/Micro Programmable Controllers Reference Manual– September 1998 | GFK-0467K |
