20.4 ADDITIONAL TOPICS

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20.4.1 Negative Logic

Some applications will use negative logic. Effectively this is the opposite of positive logic.

Positive Logic - a high/low voltage or current level indicates a true/false condition

Negative Logic - a low/high voltage or current level indicates a true/false condition

This is common in a variety of applications. Reasons include,

- we want current to flow when things are alright, consider an E-stop.

- a device input "floats high" so that when there is no input it turns on. This is common with many integrated circuits.

20.4.2 Common Logic Forms

Recognizing certain logic forms allows faster identification and solution of design problems.

These include,

- Comparitors

- Decoders

- Code conversion

- Adders/Subtractors

20.4.2.1 - NAND/NOR Forms

Remember the basic forms for NAND and NOR and then apply de'Morgan's theorem.

20.4.2.2 - Multiplexers

Multiplexors allow us to select a single output from several inputs.

In ladder logic this can be done as below,

Demultiplexers work in reverse.

20.4.2.3 - Seal-in Circuits