12/1/1981 TTL Logic LevelsContained in the article, "The Controller Even You Can Make" {Softalk, October 1981) by Silas Warner, there are two paragraphs under the label TTL Logic Levels that contain some misinformation. Perhaps the following may help. A TTL logic low level can be anywhere from 0 volts to 0.8 volts but usually is 0.4 volts or less. A TTL logic high level can be anywhere from 2.0 volts to 5.0 volts but usually is 2.4 volts or higher. There is a big difference between an output that is grounded and an output that is in the low state. When TTL parts are tested by the manufacturers an output short circuit current test is usually performed but the output is grounded for only a few hundred microseconds to prevent damaging the device. It is not a good practice to literally ground an output. Mr. Warner also discusses the differences between the output current that flows in the TTL device when high and low. It is true that TTL devices do most of their work when in the low state. This is because of the great difference between the current sinking capability (low) and current sourcing capability (high). Current flows into an output pin when it is low and out of an output when it is high.
Three-state parts are different but not considered here. The 7400 (pronounced seventy-four hundred) series TTL logic family can sink 16 milliamps (mA) and source 800 microamps (uA) in many but not all cases. Just as there are different "standard" configurations for the Apple computer, there are different "standard" TTL logic families. For instance, a 7410 TTL device and a 74LS10 (LS stands for low power Schottky) perform the same logical function but have different operating speeds, power consumption, and output drive capability. The LS part in this case can sink 8mA and source 400uA which is one half of the ratings for the 7410. There are differences between parts in the same family. A person who is putting together a circuit that uses TTL devices should refer to one of the many available TTL data books for the specifics. Kenny Lloyd, Falmouth, ME - V2N4 Comments are closed.
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