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 1/.i-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 identi fied 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-lnput 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 corre sponds 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:
Vee 4B 4A 4Y 3B 3A 3Y
lA 1B lY 2A 2B 2Y Gnd
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 V cc and is equivalent to the V symbol that I’ve been us ing to indicate a voltage. (By convention, any double letter subscript on a capi tal Vindicates a power supply. The C in this subscript refers to the collector input of a transistor, which is internally where the voltage supply is con nected.) 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 com mon ground.
For 7400 series TTL, V cc must be between 4. 7 5 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 volt age 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 O (which is no voltage). In reality, an input to one of these NAND gates can range anywhere from O volts (ground) to 5 volts (V cc). In TTL, anything between O 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 O 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 O 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 O and 3.4 volts for a logical 1, these outputs are safely within the input ranges, which are between O and 0.8 volt for a logical O and between 2 and 5 volts for a logi cal l. This is how TTL is insulated against noise. A 1 output can lose about 1.4 volts and still be high enough to qualify as a 1 input. AO output can gain 0.6 volt and still be low enough to qualify as a O input.
Probably the most important fact to know about a particular integrated circuit is the propagation time. That’s the time it takes for a change in the inputs to be reflected in the output.
Propagation times for chips are generally measured in nanoseconds, ab breviated nsec. A nanosecond is a very short period of time. One thousandth of a second is a millisecond. One millionth of a second is a microsecond. One billionth of a second is a nanosecond. The propagation time for the NAND gates in the 7400 chip is guaranteed to be less than 22 nanoseconds. That’s 0.000000022 seconds, or 22 billionths of a second.
If you can’t get the feel of a nanosecond, you’re not alone. Nobody on this planet has anything but an intellectual appreciation of the nanosecond. Nanoseconds are much shorter than anything in human experience, so they’ll forever remain incomprehensible. Every explanation makes the nanosecond more elusive. For example, I can say that if you’re holding this book 1 foot away from your face, a nanosecond is the time it takes the light to travel from the page to your eyes. But do you really have a better feel for the nano second now?
Yet the nanosecond is what makes computers possible. As we saw in Chapter 17, a computer processor does moronically simple things-it moves a byte from memory to register, adds a byte to another byte, moves the result back to memory. The only reason anything substantial gets completed (not in the Chapter 17 computer but in real ones) is that these operations occur very quickly. To quote Robert Noyce, “After you become reconciled to the nanosecond, computer operations are conceptually fairly simple.”
Let’s continue perusing the TTL Data Book for Design Engineers. You will see a lot of familiar little items in this book. The 7402 chip contains four 2-input NOR gates, the 7404 has six inverters, the 7408 has four 2-input
AND gates, the 7432 has four 2-input OR gates, and the 7430 has an 8-input NAND gate:
Yee Ne H G Ne Ne y
A B c D E F Gnd
T he abbreviation NC means no connection. T he 7474 chip is another that will sound very familiar. It’s a “Dual D-Type
Positive-Edge-Triggered Flip-Flop with Preset and Clear” and is diagrammed like this:
V cc 2Clr 2D 2Clk 2Pre 2Q 2Q
lQ lQ Gnd
The TTL Data Book even includes a logic diagram for each flip-flop in this chip:
b • Q
p ‘;:=:::L) • Q
You’ll recognize this as being similar to the diagram at the end of Chapter 14, except that I used NOR gates. T he logic table in the TTL Data Book is a little different as well:
Pre Clr Clk D Q Q
L H x x H L
H L x x L H
L L x x H* H*
H L j H H L
H H j L L H
H H L x Qo Qo
Moving right along in the TTL Data Book, you’ll discover that the 7483 chip is a 4-Bit Binary Full Adder, 74151 is a 8-Line-To-1-Line Data Selector, the 74154 is a 4-line-To-16-Line Decoder, 74161 is a Synchronous 4-Bit Binary Counter, and 74175 is a Quadruple D-Type Flip-Flop with Clear. You can use two of these chips for making an 8-bit latch.
So now you know how I came up with all the various components I’ve been using since Chapter 11. I stole them from the TTL Data Book for Design Engineers.
As a digital design engineer, you would spend long hours going through the TTL Data Book familiarizing yourself with the types of TTL chips that were available. Once you knew all your tools, you could actually build the computer I showed in Chapter 17 out of TTL chips. Wiring the chips together is a lot easier than wiring individual transistors together. But you might want to consider not using TTL to make the 64-KB RAM array. In the 1973 TTL Data Book, the heftiest RAM chip listed is a mere 256 x 1 bits. You’d need 2048 of these chips to make 64 KB! TTL was never the best technol ogy for memory. I’ll have more to say about mem