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**Home ⇒ Overview Courses ⇒ PLC ⇒ ****Combinatorial Logic**

### Circuit design using a simple example

You receive a plant description from your customer and must develop a PLC program for this purpose. Which work steps are necessary for this? This will be shown by means of a simple example.

**Example:** It should be possible to switch on a motor M1 from two control points with the latching switches S1 and S2. The motor should start when either only S1 or only S2 is switched on.

Step 1: Create a T**ruth table**

Step 2: Create a **Boolean**** equation **from the Truth table

Only the cases in which the output has the logical state '1' are interesting here! The complete Boolean equation is:

Step 3: Translate the logic equation into a **Logic circuit**

Using a PLC you might program in LD (Ladder Diagram) or FUP (Function plan). Here the solution:

Note: This logic, i.e. that the output only switches when only one input has High-signal at any one time, is frequently used in automation technology. Therefore, it has its own designation and symbol:

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