Code_ The Hidden Language of Computer Hardware and Software - Charles Petzold [94]
TTL stands for transistor-transistor logic. If in the mid-1970s you were a digital design engineer (which meant that you designed larger circuits from ICs), a 1 ¼-inch-thick book first published in 1973 by Texas Instruments called The TTL Data Book for Design Engineers would be a permanent fixture on your desk. This is a complete reference to the 7400 (seventy-four hundred) series of TTL integrated circuits sold by Texas Instruments and several other companies, so called because each IC in this family is identified by a number beginning with the digits 74.
Every integrated circuit in the 7400 series consists of logic gates that are prewired in a particular configuration. Some chips provide simple prewired gates that you can use to create larger components; other chips provide common components such as flip-flops, adders, selectors, and decoders.
The first IC in the 7400 series is number 7400 itself, which is described in the TTL Data Book as "Quadruple 2-Input Positive-NAND Gates." What this means is that this particular integrated circuit contains four 2-input NAND gates. They're called positive NAND gates because a voltage corresponds to 1 and no voltage corresponds to 0. This is a 14-pin chip, and a little diagram in the data book shows how the pins correspond to the inputs and outputs:
This diagram is a top view of the chip (pins on the bottom) with the little indentation (shown on page 250) at the left.
Pin 14 is labeled VCC and is equivalent to the V symbol that I've been using to indicate a voltage. (By convention, any double letter subscript on a capital V indicates a power supply. The C in this subscript refers to the collector input of a transistor, which is internally where the voltage supply is connected.) Pin 7 is labeled GND for ground. Every integrated circuit that you use in a particular circuit must be connected to a power supply and a common ground.
For 7400 series TTL, VCC must be between 4.75 and 5.25 volts. Another way of saying this is that the power supply voltage must be 5 volts plus or minus 5 percent. If the power supply is below 4.75 volts, the chip might not work. If it's higher than 5.25, the chip could be damaged. You generally can't use batteries with TTL; even if you were to find a 5-volt battery, the voltage wouldn't be exact enough to be adequate for these chips. TTL usually requires a power supply that you plug into the wall.
Each of the four NAND gates in the 7400 chip has two inputs and one output. They work independently of each other. In past chapters, we've been differentiating between inputs being either 1 (which is a voltage) or 0 (which is no voltage). In reality, an input to one of these NAND gates can range anywhere from 0 volts (ground) to 5 volts (VCC). In TTL, anything between 0 volts and 0.8 volt is considered to be a logical 0, and anything between 2 volts and 5 volts is considered to be a logical 1. Inputs between 0.8 volt and 2 volts should be avoided.
The output of a TTL gate is typically about 0.2 volt for a logical 0 and 3.4 volts for a logical 1. Because these voltages can vary somewhat, inputs and outputs to integrated circuits are sometimes referred to as low and high rather than 0 and 1. Moreover, sometimes a low voltage can mean a logical 1 and a high voltage can mean a logical 0. This configuration is referred to as negative logic. When the 7400 chip is referred to as "Quadruple 2-Input Positive-NAND Gates," the word positive means positive logic is assumed.
If the output of a TTL gate is typically 0.2 volt for a logical 0 and 3.4 volts for a logical 1, these outputs are safely within the input ranges, which are between 0 and 0.8 volt for a logical 0 and between 2 and