DATA COMMUNICATIONS CODES
- are prescribed bit sequence used for encoding characters and symbols.
Common Data Communication Codes
1. Baudot Code
- a simple alphanumeric code for telex machines and teleprinters which
uses 5-bit characters ( plus a start and stop bit). It is also known as the Murray Code.
- This is the International Telegraphic alphabet also known as ASCII
no.2, since five bits can only produce 32 unique codes,two codes( FIGS and LTRBS) are use
to shift between a figures function mode (with 31 characters) and a letter mode(with
another 31 characters).
- The Baudot code was developed by Thomas Murray and was named after
Emile Baudot in 1875.
| |
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|
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| Letter |
Figure Bits |
4 |
3 |
2 |
1 |
0 |
| A |
- |
1 |
1 |
0 |
0 |
0 |
| B |
? |
1 |
0 |
0 |
1 |
1 |
| C |
: |
0 |
1 |
1 |
1 |
0 |
| D |
$ |
1 |
0 |
0 |
1 |
0 |
| E |
3 |
1 |
0 |
0 |
0 |
0 |
| F |
! |
1 |
0 |
1 |
1 |
0 |
| G |
& |
0 |
1 |
0 |
1 |
1 |
| H |
# |
0 |
0 |
1 |
0 |
1 |
| I |
8 |
0 |
1 |
1 |
0 |
0 |
| J |
' |
1 |
1 |
0 |
1 |
0 |
| K |
( |
1 |
1 |
1 |
1 |
0 |
| L |
) |
0 |
1 |
0 |
0 |
1 |
| M |
. |
0 |
0 |
1 |
1 |
1 |
| N |
, |
0 |
0 |
1 |
1 |
0 |
| O |
9 |
0 |
0 |
0 |
1 |
1 |
| P |
0 |
0 |
1 |
1 |
0 |
1 |
| Q |
1 |
1 |
1 |
1 |
0 |
1 |
| R |
4 |
0 |
1 |
0 |
1 |
0 |
| S |
bel |
1 |
0 |
1 |
0 |
0 |
| T |
5 |
0 |
0 |
0 |
0 |
1 |
| U |
7 |
1 |
1 |
1 |
0 |
0 |
| V |
; |
0 |
1 |
1 |
1 |
1 |
| W |
2 |
1 |
1 |
0 |
0 |
1 |
| X |
/ |
1 |
0 |
1 |
1 |
1 |
| Y |
6 |
1 |
0 |
1 |
0 |
1 |
| Z |
" |
1 |
0 |
0 |
0 |
1 |
| Figure Shift |
|
1 |
1 |
1 |
1 |
1 |
| Letter Shift |
|
1 |
1 |
0 |
1 |
1 |
| Space |
|
0 |
0 |
1 |
0 |
0 |
| Line Speed |
|
0 |
1 |
0 |
0 |
0 |
| Blank |
|
0 |
0 |
0 |
0 |
0 |
2. ASCII YCode
- American Standard Code for Information Interchange
- ASCII NO.5 - Is a & bit standard for characters, numeral,
punctuatiion, space, carriage return, line-feed, and a few control command keys. (1977)
- The ISO adopted 7-bit ASCII as the standard ISO-646 then they've
modified it for 8 bit adding another 128 characters from the original and called it Latin
1.
- The 8 bit variation was standardized by IBM-compatibles and they call
it Extended ASCII.
3. EBCDIC
- Extended Binary Coded Decimal Interchange Code
- This is IBM's unique 8-bit answer to the international use of ASCII
code system.
- It has 256 numbers, letters and symbols. This was widely used by IBM
mainframes.
ERROR DETECTION TECHNIQUES
- Is the process of monitoring the received data and determines when a
transmission error has occurred. It does not identify the bit or bits is/are in error,
only that an error has occurred.
Three classes of probabilities
1. Class 1 : A frame arrives with no error
2. Class 2 : A frame arrives with one or more errors detected
3. Class 3 : A frame arrives with one or more errors but undetected.
TYPES OF ERROR DETECTION TECHNIQUES
1. PARITY CHECK
- The simplest bit error detection scheme is to append bit to the end
of each word in the frame.
- The idea is simple; add up the number of 1-bits in the byte( usually
a 7 bit byte), and then add an extra 1-bit or not, depending on whether the count was odd
or even. This gives you a check if one of the bits in the orginal byte becomes
accidentally changed during transmission or storage.
- A typical example is ASCII transmission, in which a parity bit is
attached to each 7-bit ASCII charater. The value of this bit is selected so that the word
has an even number of 1's( even parity) or an odd number of 1's ( odd parity
- For synchronous transmssion - odd parity : parity bit =0
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