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
1.4.2.1 - NAND/NOR Forms
• Remember the basic forms for NAND and NOR and then apply de’Morgan’s theorem.
1.4.2.2 - 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.
1.4.2.3 - Seal-in Circuits
