Teach Yourself Electricity and Electronics, 5th edition
Stan Gibilisco
Explanations for Quiz Answers in Chapter 26
1. The inclusive-OR operation, where we represent the low state as logic 0 and the high stage as logic 1, corresponds to Boolean addition. The correct choice is (c).
2. In the hexadecimal number system, the rightmost digit represents a multiple of the decimal quantity 1, while the second-rightmost digit represents a multiple of the decimal quantity 16. The numeral C in hexadecimal notation corresponds to the decimal quantity 12. In this case, the "ones" digit equals 7 while the "16s" digital equals C, or 12. Therefore, the hexadecimal numeral C7 represents the same quantity as the decimal expression

(1 x 7) + (12 x 16)
= 7 + 192
= 199

The correct choice is (b). The parentheses in the first line of the above equation aren't technically necessary (as you'll notice if you're a mathematician), but I included them to eliminate any chance of confusion.
3. In the circuit of Fig. 26-9, the inverters (NOT gates) reverse the states of the inputs X and Y. Therefore, if X = 1 then P = 0, and if Y = 0 then Q = 1. The correct choice is (b).
4. Let's work through the circuit backwards. In order to obtain a high state at the output of the circuit shown in Fig. 26-9, we must have R = 1. The output comes from an AND gate, which requires high states at both inputs in order to generate a high state at the output. Therefore, R = 1 only if P = 1 and Q = 1. Note that P represents the negation of X while Q represents the negation of Y. If we want both P and Q to equal logic 1 (the high state), both inputs X and Y must equal logic 0 (the low state). The correct choice is (d).
5. In the binary numeration system, the rightmost digit represents multiples of the decimal quantity 20, or 1. We have a five-digit binary numeral in this situation. Proceeding to the left:
  • The second digit from the right represents multiples of the decimal quantity 21, or 2
  • The third digit from the right represents multiples of the decimal quantity 22, or 4
  • The fourth digit from the right represents multiples of the decimal quantity 23, or 8
  • The fifth digit from the right represents multiples of the decimal quantity 24, or 16

Our binary numeral is 10101. Based on the above-described scheme, its decimal equivalent is

(1 x 16) + (0 x 8) + (1 x 4) + (0 x 2) + (1 x 1)
= 16 + 0 + 4 + 0 + 1
=21

The correct choice is (b).
6. In the octal system, the rightmost digit represents multiples of the decimal quantity 80, or 1. We have a three-digit octal numeral here. Proceeding to the left:
  • The second digit from the right represents multiples of the decimal quantity 81, or 8
  • The third digit from the right represents multiples of the decimal quantity 82, or 64

We're given the octal quantity 425. Based on the aforementioned information, its decimal equivalent is

(4 x 64) + (2 x 8) + (5 x 1)
= 256 + 16 + 5
= 277

The correct choice is (c).
7. By definition, in parallel data transmission we send bits in batches along multiple lines or channels. The correct answer is (d).
8. Let's work through this circuit from the output to the input. We want to obtain logic 0 (the low state) at the output point Q of the black box diagrammed in Fig. 26-10. That means we want logic 1 (the high state) to exist at point P, just before the inverter but after the XOR gate. From the text, we recall that the output of an XOR gate equals 1 (high) if and only if the input states oppose each other. In this case, that means either X = 0 and Y = 1, or else X = 1 and Y = 0. The correct choice is (d).
9. We want logic 1 (high) at the output Q of the circuit in Fig. 26-10. Let's work through the circuit step-by-step, from the output to the input, to see what the inputs must be. In order to get Q = 1, we must have P = 0. The output of an XOR gate equals 0 if and only if the input states coincide. In this case, that means either X = 0 and Y = 0, or else X = 1 and Y = 1 -- in other words, X = Y. The correct choice is (c).
10. The second digit from the left in an eight-digit binary numeral represents the seventh digit from the right. Extending the explanation for the answer to Question 5 above, we have the following description of the digit values:
  • The sixth digit from the right represents multiples of the decimal quantity 25, or 32
  • The seventh digit from the right represents multiples of the decimal quantity 26, or 64

The correct choice is (a).
11. The term baud refers to the number of times per second that a binary signal changes state (from low to high-or vice-versa). We won't encounter this term often, although we might see it in the literature for an old communications system. The correct choice is (c).
12. When both inputs of an R-S flip-flop are high, the output states are difficult to predict. The correct choice is (a).
13. In the circuit of Fig. 26-11, the output R emerges from an XOR gate, which requires opposite states at the inputs to attain a high state at the output. Therefore, R = 1 only if the logic states at P and Q differ. In this circuit, P represents the negation of X while Q represents the negation of Y. If we want the logic states at P and Q to oppose each other, then the logic states at X and Y must also oppose each other. The correct choice is (d).
14. Once again, let's work through the system backwards, starting at the output. We want to have R = 0. In an XOR gate, the output is low if and only if the input states agree. That means we must see P = Q. Because we have NOT gates at both inputs, we'll see P = Q if and only if X = Y, that is, when the input states agree. The correct choice is (c).
15. In the hexadecimal system, we denote the single digit following 9 as an uppercase letter A (the equivalent of the decimal quantity 10). Therefore, as we count upward in hexadecimal terms, we get 99A immediately after 999. The correct choice is (b).
16. When we talk about bits (or bits per second), the prefix "kilo-" means 103, or 1000. When we talk about bytes, the prefix "kilo-" means 210, or 1024. The correct choice is (a).
17. In the binary numeration system, the rightmost digit represents multiples of the decimal quantity 20, or 1. Proceeding to the left:
  • The second digit from the right represents multiples of the decimal quantity 21, or 2
  • The third digit from the right represents multiples of the decimal quantity 22, or 4
  • The fourth digit from the right represents multiples of the decimal quantity 23, or 8
  • The fifth digit from the right represents multiples of the decimal quantity 24, or 16
  • The sixth digit from the right represents multiples of the decimal quantity 25, or 32
  • The seventh digit from the right represents multiples of the decimal quantity 26, or 64
  • The eighth digit from the right represents multiples of the decimal quantity 27, or 128

If all the digits in an eight-digit binary number are 1, then the sum total in decimal terms equals 255. That's the largest possible decimal value that we can represent as an eight-digit binary numeral. The correct choice is (b).
18. If we shop for a new computer and accessories these days, we'll rarely, if ever, encounter a diskette (or "floppy") drive. The correct choice is (c). Most computers contain built-in DVD-RW drives (a), and most also contain built-in hard drives (b). Thumb drives (d), technically called flash memory modules, are sold along with computers and can be plugged directly into one of the USB ports.
19. When both inputs to a two-input NAND gate equal logic 0 (the low state), the output is high. The correct choice is (b).
20. In an e-mail address, the final few characters (usually two, three, or four) after the last period tell us the type of domain with which the address is associated. The correct choice is (d).