Data Communications Prepared By Phil Marcello Data Comm Network BBS Rochester. N.Y.14611 716-328-3844 Chapter 1.0 Intro .1 Basic Circuit .2 Bits & Bytes .3 ASCII Chapter 2.0 The Interface .1 Parallel Interface .2 Serial Interface .3 Parity & More .4 Bit Length .5 Data Blocks .6 Protocols .7 The UART Chapter 3.0 More on the Interface .1 Control Signal .2 DCE & DTE .3 Sync & Async Chapter 4.0 Electrical characteristics Chapter 5.0 Modulation .1 Phase Modulation Layman's Terms .2 Phase Modulation The Tech side .3 Introduction of Clocking Chapter 6.0 Introduction to Telephone Lines Chapter 7.0 The switched Communications Network Welcome to the world of Data Communications. A world of fast moving technical innovation. Even as this course is being presented, the technology is leaping forward. The material in this course is presented in such a way as to hopefully be of value to people of differing backgrounds and levels of technical expertise. Just who is Phil Marcello ?? Nobody of great consequence. Just a hard working Senior Field Engineer for General Data Comm (6 years) with 5 years prior service with Racal Milgo, both relative giants in the communications industry. My experience is mostly in the commercial area, high speed synchronous communication. To list my weaknesses would fill your hard drive. My reference material at the moment but subject to expansion include Bell System Technical References "Data Comm Using The Switched Telecommunications Network" "Data Comm Using VoiceBand Private Line Channels" Technical Aspects of Data Communications by John E. McNamara The Modem Reference by Michael A. Banks Technical Package For the Tellabs Registered Network Interface System A General Data Comm publication Warwick Networking Products Catalog Chapter One - Some Basics 1.1 Basic Circuit For those of you out there, who know how to type on a keyboard and run a program but nothing else, the following basics are for you. The flashlight represents the most basic electrical circuit. A battery - A switch - and a lightbulb. When the switch is closed, electrons are allowed to flow and the lightbulb lights. This is an "ON" condition. When the switch is opened, the circuit path is broken. Electrons can not flow, the lightbulb is "OFF" With just this example, we have created a 2 state circuit; On and OFF. Let the numeral 1 represent the on condition and 0 represent the off condition and we now have a simple way of expressing what the circuit is trying to tell us. This circuit can be used to provide information to tell us something. When the light is on "Do Not Enter The Room" is an example. This is Information. Information is Data. Conveying information is Data Communications. Instead of a light bulb, we feed the "on" (1) or "off" (0) into a logic circuit. This is the bassis for all computer operation. Data becomes more valuable as more people have access to it. Data stored at one location and available only to people at that location, limits the potential value of that Data. Providing access to that data by people all over the country is called Data Communications. The transfer of information inside a computer is usually handled via a Parallel Bus. If one lightbulb circuit can provide us with 2 states of information then 8 light bulbs operating side by side can give us 8 channels of Data simultaneously. Eight lights Bulbs flashing 1's and 0's conveying Coded information to any onlooker. 1.2 Bits & Bytes One channel, One on/off circuit, provides us with one BIT of Data. A "0" (zero) bit or a "1" (one) bit. When we put bits of data together, we get a BYTE. The term byte is interpreted differently by different people. A BYTE is a predetermined number of BITS. We can say that our BYTE will be equal to 8 BITS. Thus our 8 circuits provide us with one BYTE. 00101010 is 8 bits and equals one byte Or we could say that our BYTE is equal to 4 Bits. Then we would have to separate our 8 circiuts. We would then say circuits 1,2,3 and four make up one BYTE (of 4 bits) and circuits 5,6,7 and 8 make up another BYTE of four bits. To get the same effect as one 8 bit BYTE we would eventually have to put them together anyways. We do this ; putting smaller BYTES together ; by creating a WORD. 0101 1011 are two, four bit bytes making a WORD So we can either say we have a BYTE of 8 bits OR we can say we have a WORD consisting of 2 BYTES at 4 bits per BYTE. Since the terms are used differently by different authorities, I have selected these terms for our purpose. Back to our 8 circuits. Since these circuits provide us with data, it is time we called them by their correct name. Each circuit is a DATA CHANNEL. They are designated Data Channel or Channel 1, 2, 3 ... etc. The information on these Channels are kept in Sync by a common clock that allows the 8 data bits to enter the channel and leave the channel at the same time. Where these channels connect to inside the computer is better left to a computer course. I also chave no desire at this point to go into Boolean Algebra or Binary Arithmetic. Suffice to say, that just as the Morse Code uses a combination of Dih's and Dah's to represent letters of the alphabet, we use combinations of 1's and 0's to represent Alpha characters (letters), Numerical Codes (numbers), Punctuation and control characters. 1.3 ASCII Code By combining the 8 characters in our Bus together we have 8 channels of 2 states each giving us 256 possible combinations. HOW !!! Just count all the different possible combinations that 8 channels of 1's and 0's can provide for us. 00000000 00000100 00000001 00000101 00000010 00000110 etc etc etc all the way 00000011 00000111 up to 11111111 On the next page is provided the means for using these 1's and 0's as information, however to make ourt life easier, we will be using WORD format, 2 BYTES at 4 bits per byte. What is needed now is to turn those 1's and 0's into characters that we can understand. Well someone did it for us and they created a standard list called American Standard Code for Information Interchange more affectionately known as ASCII (pronounced ASSKEY) ASCII TABLE 8 Bit (Octal is provided for those who need it) Binary Alpha Octal Code Numerics Code 0000 0000 NULL 000 0001 SOH 001 0010 STX 002 0011 ETX 003 0100 EOT 004 0101 ENQ 005 0110 ACK 006 0111 BEL 007 0001 0000 BS 10 0001 HT 11 0010 LF 12 0011 VT 13 0100 FF 14 0101 CR 15 0110 SO 16 0111 SI 17 0010 0000 DLE 20 0001 DC1 21 0010 DC2 22 0011 DC3 23 0100 DC4 24 0101 NAK 25 0110 SYN 26 0111 ETB 27 0011 0000 CAN 30 0001 EM 31 0010 SUB 32 0011 ESC 33 0100 FS 34 0101 GS 35 0110 RS 36 0111 US 37 0100 0000 SP 40 0001 ! 41 0010 " 42 0011 # 43 0100 $ 44 0101 % 45 0110 & 46 0111 ' 47 0101 0000 ( 50 0001 ) 51 0010 * 52 0011 + 53 0100 , 54 0101 - 55 0110 . 56 0111 / 57 0110 0000 0 60 0001 1 61 0010 2 62 0011 3 63 0100 4 64 0101 5 65 Binary Alpha Octal Code Numerics Code 0110 6 66 0111 7 67 0111 0000 8 70 0001 9 71 0010 : 72 0011 ; 73 0100 < 74 0101 = 75 0110 > 76 0111 @ 77 1000 0000 A 100 0001 B 101 0010 C 102 0011 D 103 0100 E 104 0101 E 105 0110 F 106 0111 G 107 1001 0000 H 110 0001 I 111 0010 J 112 0011 K 113 0100 L 114 0101 M 115 0110 N 116 0111 O 117 1010 0000 P 120 0001 Q 121 0010 R 122 0011 S 123 0100 T 124 0101 U 125 0110 V 126 0111 W 127 1011 0000 X 130 0001 Y 131 0010 Z 132 0011 [ 133 0100 \ 134 0101 ] 135 0110 ^ 136 0111 _ 137 1100 0000 ` 140 0001 a 141 0010 b 142 0011 c 143 0100 d 144 0101 e 145 0110 f 146 0111 g 147 1101 0000 h 150 0001 i 151 0010 j 152 0011 k 153 0100 l 154 0101 m 155 0110 n 156 0111 o 157 Binary Alpha Octal Code Numerics Code 1110 0000 p 160 0001 q 161 0010 r 162 0011 s 163 0100 t 164 0101 u 165 0110 v 166 0111 w 167 0000 x 170 0001 y 171 0010 z 172 0011 { 173 0100 | 174 0101 } 175 0110 ~ 176 0111 DEL 177