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Scan time of the programmable controller
• The process of input update, program execution, and output update is referred to as a
scan
and the process
repeated over and over in the same manner is referred to as the
cyclic execution method.
• In the cyclic execution method, since the process of input update is performed immediately after the output
update, the process of input update and output update is sometimes called
I/O update
for the purpose of
simplification.
• In addition to program execution and I/O update, the programmable controller also performs a variety of error
checking (self-diagnostic function) and also communicates with the programming tools. These operations are
referred to, as a whole, as
tool services
and are performed after program execution.
• Since the
scan time
is defined as the time required for one scan, the cyclic operation of a scan (I/O update,
program execution, and tool service) can be shown below.
Program
execution Program
execution
Scan time Scan time
I/O update
Tool services
I/O update
Tool services
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5-1. Operating Principles of the Programmable Controller