Now that we are familiar with basic logic gates, it is important to understand that our observations were guided completely according to the positive true logic. That means that we concentrated on getting high positive output as a result of the gates, mostly because we have shown output on the LED. But, in reality, it is often quite the contrary, since TTL chips are much better at sinking current than at sourcing it. In other words, TTL designers mostly use negative true logic, using low negative value as an output for true conditions, in order to drive other chips that use negative values as inputs. In that sense, it is very important to follow DeMorgan’s transformations, which enable us to understand the negative versions of our basic logic gates. For example, positive logic NAND gate can be expressed also as negative logic OR gate – if any of the inputs is low, then its inverted value becomes high input into OR gate and the result is high. As we will see, sometimes it is even easier to understand the gate by looking at it from a negative perspective.
(Source: referenced)
Note: The missing transformation of the XOR gate will be explained later in the circuits section.
References:
Dr. Hugo Holden – Lawn Tennis: https://www.worldphaco.com/uploads/LAWN_TENNIS.pdf
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