229
The overall flow of CPU operation is as shown in the following flowchart:
YES
NO
YES
YES
NO
NO
Power application
Clears IR area and
resets all timers
Checks I/O Unit
connections
Resets watchdog
timer
Checks hardware and
Program Memory
Check OK?
Refresh output signals
according to output bit
status
Process Remote I/O Units
Resets watchdog timer
Services Peripheral
Devices and Link Units
End of program?
ERROR or ALARM
Sets error flags and
lights indicator
Resets watchdog timer and
program address counter
Executes program
End of program?
Resets watchdog timer
Refreshes input bits
according to input signals
ERROR
ALARM
Input
refreshing
Peripheral Device and Link Unit servicing
Program execution
PC scan time
Remote I/O
processing
Output
refreshing
Overseeing
processes
Initialization
on power-up
The first three operations immediately after power application are performed
only once each time the PC is turned on. The rest of the operations are per-
formed in cyclic fashion, with each cycle forming one cycle. The cycle time is
Cycle Time
Section 6-1
