and RS232 Serial Communications
Data in a computer, at the hardware level, is represented as binary, two state, electrical signals usually referred to as bits. For two devices to exchange information (data) a connection must exist and the senders and the receivers must agree upon how the data will be represented and interpreted in advance. One of the most prominent representations is the American Standard Code for Information Interchange or ASCII. ASCII assigns a unique binary representation for specific characters and symbols
ASCII Table (page 78)
Decimal 000
001 002 003 004 005 006 007 008 009 010 011 012 013 014 015 016 017 018 019 020 021 022 023 024 025 026 027 028 029 030 031 032 033 034 035 036 037 038 039 040 041 042
Binary
0000000 0000001 0000010 0000011 0000100 0000101 0000110 0000111 0001000 0001001 0001010 0001011 0001100 0001101 0001110 0001111 0010000 0010001 0010010 0010011 0010100 0010101 0010110 0010111 0011000 0011001 0011010 0011011 0011100 0011101 0011110 0011111 0100000 0100001 0100010 0100011 0100100 0100101 0100110 0100111 0101000 0101001 0101010
Symbol (Null)
SOH STX ETX EOT ENQ ACK BEL BS HT LF VT FF CR SO SI DLE DC1 DC2 DC3 DC4 NAK SYN ETB CAN EM SUB ESC FS GS RS US (Space) ! " # $ % & ' ( ) *
Decimal
043
Binary
0101011 0101100 0101101 0101110 0101111 0110000 0110001 0110010 0110011 0110100 0110101 0110110 0110111 0111000 0111001 0111010 0111011 0111100 0111101 0111110 0111111 1000000 1000001 1000010 1000011 1000100 1000101 1000110 1000111 1001000 1001001 1001010 1001011 1001100 1001101 1001110 1001111 1010000 1010001 1010010 1010011 1010100 1010101
Symbol
+ , . /
044 045 046 047 048 049 050 051 052 053 054 055 056 057 058 059 060 061 062 063 064 065 066 067 068 069 070 071 072 073 074 075 076 077 078 079 080 081 082 083 084 085
0 1 2 3 4 5 6 7 8 9 : ; < = > ? @ A B C D E F G H I J K L M N O P Q R S T U
Decimal 086 087 088 089 090 091 092 093 094 095 096 097 098 099 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127
Binary
1010110 1010111 1011000 1011001 1011010 1011011 1011100 1011101 1011110 1011111 1100000 1100001 1100010 1100011 1100100 1100101 1100110 1100111 1101000 1101001 1101010 1101011 1101100 1101101 1101110 1101111 1110000 1110001 1110010 1110011 1110100 1110101 1110110 1110111 1111000 1111001 1111010 1111011 1111100 1111101 1111110 1111111
Symbol V W X Y Z [ \ ] ^ _ ` a b c d e f g h i j k l m n o p q r s t u v w x y z { | } ~ DEL
Serial Communications For serial data communications the bits of a character are sent and received one at a time, For example the character is Y coded using ASCII as 1011001
LSB 1 0 0 1 1 0 MSB 1
To transmit the character Y the sender sends the string of bits 1011001 as a serial sequence of high and low voltage levels, relative to some baseline reference typically zero volts.
RS232 Standard
RS232 is the recommended standard for connecting computers and peripherals via serial interfaces for the purpose of communication. The standard defines a set of rules governing the connections and communication. The standard dictates that: 1. 2. 3. 4. Data signal amplitudes occur between +/-15 volts. A voltage level between -15 and -3 volts is a binary bit 1 A voltage level between +3 and +15 volts as a binary bit 0 The voltage levels between +/-3 volts is undefined.
RS232 representation of the character Y using the ASCII code
LSB MSB
1
0
0
1
1
0
1
+V Logic 0
0V
Logic 1
-V
Serial transmissions include a start bit and a stop bit that encapsulate the actual data bits. The start bit lets the receiver know that data bits will follow. The stop bit indicates the end of the transmission of a particular character or symbol.
Start bit LSB MSB Stop bit
0
1
0
0
1
1
0
1
1
+V Logic 0
0V
Logic 1 -V
Serial Data signals are susceptible to noise and electrical interference Causing unintentionally data flips from 0 to 1 or from 1 to 0, or error bits. One of the simplest techniques for detecting bit errors is the parity check method. In the parity check scheme the sender makes the number of 1s representing an ASCII character or symbol, odd or even by appending a redundant bit at the end ...