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 Truth table
Truth 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:
Boolean equation
Step 3: Translate the logic equation into a Logic circuit
Logic circuit
Using a PLC you might program in LD (Ladder Diagram) or FUP (Function plan). Here the solution:
Learning PLC - XOR in LD and FUP
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:
XOR, Exclusive-Or
Example: Two out of Three Logic with Boolean Algebra
An injection molding machine may only start or continue the production process when the required operating temperature is reached and maintained. As a production stop of this installation during operation is involved with high costs, the temperature is measured by three sensors. The shut-down should only take place if at least two of the three temperature sensors report a temperature error (signal '1'). In this manner, unnecessary shut down can be prevented, which would cause high costs.
Two out of Three Logic with Boolean Algebra
- Develop for this system the necessary simplified ladder diagram.
