Positive and negative Logic
To this point, we have been dealing with one type of LOGIC POLARITY, positive. Let’s further define logic polarity and expand to cover in more detail the differences between positive and negative logic.
Logic polarity is the type of voltage used to represent the logic 1 state of a statement. We have determined that the two logic states can be represented by electrical signals. Any two distinct voltages may be used. For instance, a positive voltage can represent the 1 state, and a negative voltage can represent the 0 state.
The opposite is also true. Logic circuits are generally divided into two broad classes according to their polarity, positive logic and negative logic. The voltage levels used and a statement indicating the use of positive or negative logic will usually be specified on logic diagrams supplied by manufacturers.
In practice, many variations of logic polarity are used; for example, from a high-positive to a low positive voltage, or from positive to ground; or from a high-negative to a low-negative voltage, or from negative to ground.
A brief discussion of the two general classes of logic polarity is presented in the following paragraphs.
Positive logic is defined as follows: If the signal that activates the circuit (the 1 state) has a voltage level that is more POSITIVE than the 0 state, then the logic polarity is considered to be POSITIVE.
The table below shows the manner in which positive logic may be used. As you can see, in positive logic the 1 state is at a more positive voltage level than the 0 state.
Examples of positive logic.
As you might suspect, negative logic is the opposite of positive logic and is defined as follows: If the signal that activates the circuit (the 1 state) has a voltage level that is more NEGATIVE than the 0 state, then the logic polarity is considered to be NEGATIVE.
The next table illustrated below shows the manner in which negative logic may be used. NOTE: The logic level LOW now represents the 1 state. This is because the 1 state voltage is more negative than the 0 state.
In the examples shown for negative logic, you notice that the voltage for the logic 1 state is more negative with respect to the logic 0 state voltage. This holds true in example 1 where both voltages are positive. In this case, it may be easier for you to think of the TRUE condition as being less positive than the FALSE condition. Either way, the end result is negative logic.
The use of positive or negative logic for digital equipment is a choice to be made by design engineers. The difficulty for the technician in this area is limited to understanding the type of logic being used and keeping it in mind when troubleshooting.
Examples of negative logic.
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