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Automating Manufacturing Systems - Process Control and ...

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plc sfc - 20.12<br />

13<br />

reset<br />

button<br />

automatic<br />

button<br />

8<br />

2<br />

15<br />

3 power on<br />

light on<br />

part not<br />

detected<br />

part detect<br />

10<br />

4 advance on<br />

part hold on<br />

12 stop<br />

button<br />

7 light off<br />

advance off<br />

power off<br />

11<br />

14<br />

bottom<br />

limit<br />

5 advance off<br />

retract on<br />

top<br />

limit<br />

6 retract off<br />

part hold off<br />

Figure 20.10<br />

SFC Renumbered<br />

Some of the ladder logic for the SFC is shown in Figure 20.11. Each program corresponds<br />

to the number on the diagram. The ladder logic includes a new instruction, EOT,<br />

that will tell the PLC when a transition has completed. When the rung of ladder logic with<br />

the EOT output becomes true the SFC will move to the next step or transition. when developing<br />

graphical SFCs the ladder logic becomes very simple, <strong>and</strong> the PLC deals with turning<br />

states on <strong>and</strong> off properly.

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