GFK-0467K Chapter 2 System Operation 2-3

2
All of these steps execute every sweep. Although the Programmer Communications Windowopens each sweep, programmer services only occur if a board fault has been detected or if theprogramming device issues a service request; that is, the Programmer Communications Windowfirst checks for work to do and exits if there is none. The sequence of the standard program sweepis shown in the following figure.Figure 2-1. PLC Sweep
SYSTEM
COMMUNICATIONS
I/O
?
ENABLED
RUN
?
MODE
a43064
START-OF-SWEEP
START NEXT SWEEP
INPUT SCAN
USER PROGRAM
PROGRAMMER
OUTPUT SCAN
I/O NO
LOGIC SOLUTION
NO
NO
YES
YES
YES
HOUSEKEEPING
DATA
PROGRAM
DATA
PROGRAMMER
DIAGNOSTICS
SCAN
PLC
OF
TIME
SYSTEM
HOUSEKEEPING
?
ENABLED
EXECUTION
INPUT
OUTPUT
COMMUNICATIONS SERVICE
COMMUNICATIONS
CALCULATION
CHECKSUM