1.4.1 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.



1.4.2 Common Logic Forms


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


• These include,

- Comparitors

- Decoders

- Code conversion

- Adders/Subtractors - NAND/NOR Forms


• Remember the basic forms for NAND and NOR and then apply de’Morgan’s theorem.
 - 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. - Seal-in Circuits