97
YES
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?
Resets watchdog timer
Services Peripheral
Devices
ERROR or ALARM
Sets error flags and
lights indicator
Executes program
Input and output
refreshing
ERROR
ALARM
Resets watchdog timer
The first three operations immediately after power application are performed
once each time the PC is turned on. The then on the operations shown
above are performed in cyclic fashion, with each cycle forming one scan. The
scan time is the time that is required for the CPU to complete one of these
cycles. This cycle includes four types of operation.
1. Overseeing
2. Input/Output refreshing
3. Peripheral Device servicing
4. Instruction execution
Scan time = Overseeing time + Input/output refreshing + Peripheral
Device servicing time + Instruction execution time
1, 2, 3...
Scan Time Section 6–2
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