FC6A S
ERIES
MICROS
MART
L
ADDER
P
ROGRAMMING
M
ANUAL
FC9Y-B1726 A-5
A
PPENDIX
Processing in One Scan
While the FC6A Series MICROSmart is running, the FC6A Series MICROSmart performs operations repeatedly such as input
refreshing, ladder program processing, and error checking.
A
scan
is the execution of all instructions from address zero to the END instruction. The time required for this execution is referred
to as one
scan time
. The scan time varies with respect to program length.
The current value of the scan time is stored to special data register D8023 (scan time current value), and the maximum value of
the scan time is stored to special data register D8024 (scan time maximum value). These values can be viewed on the PLC status
dialog box of WindLDR while monitoring on a computer.
Executing Program Instructions
During the scan time, program instructions are processed sequentially starting with the first line of the ladder program, except for
interrupt program execution. One scan time of a ladder program is approximately equal to the total execution time of each
instruction shown on preceding pages.
Watchdog Timer
The watchdog timer monitors the time required for one program cycle (scan time) to prevent hardware malfunction. When the
processing time for one scan exceeds the allowed time, a watchdog timer error occurs and the system is reset. If this is the case,
place NOP instructions in the ladder diagram. The NOP instruction resets the watchdog timer. For details on the watchdog timer,
see Chapter 5 "Watchdog Timer Settings" in the "FC6A Series MICROSmart User’s Manual".
Breakdown of END Processing Time
The END processing time depends on the FC6A Series MICROSmart settings and system configuration. The maximum values for
the END processing time are as follows.
Item Processing Time
END processing 640 µs
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