英语_电子信息工程3.pptVIP

　　A general method for converting from a decimal to a binary number is indicated in Table 3.3. The procedure is the following. Place the decimal number (in this illustration, 19) on the extreme right. Next divide by 2 and place the quotient (9) to the left and indicate the remainder (1) directly below it. Repeat this process (for the next column 9÷2 =4 and a remainder of 1) until a quotient of 0 is obtained. The array of 1’s and 0’s in the second row is the binary representation of the origin decimal number. In this example, decimal 19=10011 binary. 　　A binary digit (a 1 or a 0) is called a bit. A group of bits that has the same significance is called a byte, word, or code. For example, to represent the 10 numerals (0, 1, 2, …, 9) and the 26 letters of the English alphabet would require 36 different combinations of 1’s and 0’s. Since 253626, then a minimum of 6 bits per bite are required in order to accommodate all the alphanumeric characters. In this sense a bite is sometimes referred to as a character and a group of one or more characters as a word. 　　2. Logic Systems 　　In a DC, or level-logic, system a bit is implemented as one of two voltage levels. If, as in Figure 3.6(a), the more positive voltage is the 1 level and the other is the 0 level, the system is said to employ DC positive logic. On the other hand, a DC negative-logic system, as in Figure 3.6(b), is one which designates the more negative voltage state of the bit as the 1 level and the more positive as the 0 level.[1] It should be emphasized that the absolute values of the two voltages are of no significance in these definitions. In particular, the 0 state need not represent a zero voltage level (although in some systems it might). Figure 3.6 Illustrating the definition of (a) positive and (b) negative logic. A transition from one state to the other occurs at t=t 　　The parameters of a physical device (for example, VCE·sat of a transistor) are not identical from sample to sample, and they also