From the above examples, you can see that a system using positive logic may not even use a
positive signal. A positive logic system is one that is classified or defined as a system in which the more
positive signal represents binary 1.
Since positive logic is defined as a system using the more positive of two signals to represent
binary 1, it follows that negative logic must be a logic system that uses the more negative of two signal
voltages to represent binary 1.
The more negative of two voltages such as -6 and -12 is obviously the -12 and thus it represents
binary 1. In a system using +6 and +12 volts, the +6 is the more negative of the two and represents
Learning Event 3:
IDENTIFY STATE INDICATOR AND INVERTER SYMBOLS
There are many different pairs of voltages in use and in some applications, such as
communications, the levels are different in each piece of equipment. Therefore, common titles are used
to identify the two signals' conditions regardless of their actual levels. The system we will discuss here
is the most popular one and the least confusing. The two signal levels are simply called high and low,
and are the same as 1 and 0.
a. The more positive of the two voltages is the high (H) and the more negative of the two
voltages is the low (L). This method has a great advantage in that it does not matter what the two
voltages are, one is always more positive than the other. Even if signal levels change from one
equipment to the other, the high and low concept can still be applied.
b. The method also allows us to discuss symbols and the circuit functions they represent,
without regard to which voltage represents a binary 1 and which voltage represents a binary 0.
The military standard logic uses small circles called state indicators at the inputs and outputs of
the logic symbols. The lines entering and leaving a symbol represent its input and output, the presence
or the absence of a circle on each line specifies what the active signal condition is for that connection.
a. The presence of a circle indicates that at that point in the circuit the relatively low (L) voltage
is the active state of the signal. A circle on an input to a symbol means that a low is required to activate
or "turn on" that circuit. A circle on the output means that the output is a low when the circuit is
activated or "turned on."
b. The absence of a circle indicates that at that point in the circuit the relatively high (H) voltage
is the active state of the signal. The absence of a circle on an input to a symbol means that a high is
required to activate or "turn-on" the circuit. Absence of a circle on the output means that the output is
high when the circuit is activated or "turned on."