Data Communication Codes
DATA CODES
This refers to the way in which data is represented. The sender and receiver must use the same code in order to communicate properly. Here, we will briefly look at two common codes, one which was developed earlier on and was widely used in early telegraph systems, and the other, which is in widespread use today.
The Baudot Code
The Baudot code was used extensively in telegraph systems. It is a five bit code invented by the Frenchman Emile Baudot in 1870. Using five bits allowed 32 different characters. To accommodate all the letters of the alphabet and numerals, two of the 32 combinations were used to select alternate character sets.
Each character is preceded by a start bit, and followed by a stop bit. It is an asynchronous code, and thus suited for low speed data communication.
For instance, lets consider coding the phase “JAMES BOND 007 SAYS HI!” using the Baudot code. To switch between the LTRs and FIGs requires the use of a LetterShift or a FigureShift. Once switched, you stay in that mode till you want to switch back again. So, here is the phrase encoded in Baudot.
| J | A | M | E | S | B | O | N | D | 0 | 0 | 7 | S | A | Y | S | H | I | ! | ||||
| 31 11 | 3 | 28 | 1 | 5 | 4 | 25 | 24 | 12 | 9 | 4 | 27 22 | 22 | 7 | 4 | 31 5 | 3 | 21 | 5 | 4 | 20 | 6 |
27 13 |
ASCII (American Standard Code for Information Interchange)
The ASCII code is the most popular code for serial data communications today. It is a seven bit code (128 combinations), and thus supports upper and lowercase characters, numeric digits, punctuation symbols, and special codes. The table below lists the values for each character in the ASCII set.
| 00 | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 | 0A | 0B | 0C | 0D | 0E | 0F | |
| 00 | NUL | SOH | STX | ETX | EOT | ENQ | ACK | BEL | BS | TAB | LF | VT | FF | CR | SO | SI |
| 10 | DLE | DC1 | DC2 | DC3 | DC4 | NAK | SYN | ETB | CAN | EM | SUB | ESC | FS | GS | RS | US |
| 20 | ! | “ | # | $ | % | & | ‘ | ( | ) | * | + | , | – | . | / | |
| 30 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | : | ; | < | = | > | ? |
| 40 | @ | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O |
| 50 | P | Q | R | S | T | U | V | W | X | Y | Z | [ | \ | ] | ^ | _ |
| 60 | ` | a | b | c | d | e | f | g | h | i | j | k | l | m | n | o |
| 70 | p | q | r | s | t | u | v | w | x | y | z | { | | | } | ~ | DEL |
To work out a particular value from the table, you first determine the row value, then add the column value. For example, the character A has a value of 41, being a row value of 40 and a column value of 1.
ASCII is also used as the data code for keyboards in computers. Control Codes have values between 00 and 1F (hexadecimal). Control codes are used in binary synchronous communication, and device control codes in communicating with devices such as printers or terminals.
A control code can be generated from a keyboard by holding down the Ctrl key and pressing another key. For instance, holding down the Ctrl key and pressing the A key generates the control code SOH.
