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Automating Manufacturing Systems - Process Control and ...

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plc advanced functions - 16.19<br />

• The normal operation of the PLC is<br />

fast [input scan]<br />

Input values scanned<br />

slow [ladder logic is checked]<br />

Outputs are updated in<br />

memory only, as the<br />

ladder logic is scanned<br />

fast [outputs updated]<br />

Output values are<br />

updated to match<br />

values in memory<br />

Figure 16.19<br />

Input, Program <strong>and</strong> Output Scan<br />

Figure 16.20 shows a segment within a program that will update the input word<br />

I:001, determine a new value for O:010/01, <strong>and</strong> update the output word O:010 immediately.<br />

The process can be repeated many times during the program scan allowing faster<br />

than normal response times.

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