here is plc program examples about siemens s7-300 plc , You can learn about Timer instruction。
plc ladder diagram
(1)Control requirements.
the operator can set the heating time with a dial switch, and the operator set the value to be displayed in a second unit in a BCD format.
(2) Defining the programming element symbol address
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System components
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address
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Start button
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I0.7
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Single digit dial switch
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I1.0～I1,3
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Tens digit dial switch
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I1.4～I1.7
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Hundreds digit dial switch
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I0.0～I0.3
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Start heating
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Q4.0
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(3) Ladder diagram
PLC ONE 