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USER'S GUIDE
DATA TRANSMISSION CODES
Applications
Baudot/Murray code is still the most widely used RTTY code in amateur radio, and this will prob-
ably continue for some years to come. Even with its limitations, Baudot still has some advantages
for the amateur operator.
ASCII – AMERICAN STANDARD CODE FOR INFORMATION INTERCHANGE
ASCII is an extension of the eight-bit code developed decades ago and used for years in the Bell
System TWX network. ASCII was published as a standard by ANSI, the American National Stand-
ards Institute around 1963, in order to achieve some degree of compatibility for the newborn data
communications field. ASCII is also known as International Alphabet Number 5, and is standar-
dized for international traffic at various data rates.
ASCII uses seven bits to define each character. That give us a maximum of 27, or 128 possible
combinations. We now have the ability to transmit upper case and lower case letters, all punctua-
tions, ten digits and control codes such as 'start of text', 'end of text', 'horizontal tab', 'vertical tab',
'form feed', 'backspace' and many other control functions unavailable in Baudot.
ASCII is not an eight-bit code; the eighth bit is reserved for calculating parity – a form of error de-
tection. However, many ASCII systems do not require the eighth bit for parity and operate well
without it.
OTHER DATA CODES
Baudot code was fine in the days when all communication was between humans. But as technol-
ogy developed machines started to communicate with other machines without involving humans.
Better and more efficient codes had to be developed for transmitting information so that the ma-
chines themselves could evaluate the received information and request repeats if errors were re-
ceived.
As data processing first evolved and data communication codes were developed, the data process-
ing systems used their own codes. One of the first of these was 'BCD', Binary Coded Decimal. BCD
was used for internal calculations inside a data processing device. BCD has no alpha characters,
only numbers, and thus was unsuitable as a communications code for use by humans.
BCDIC
Binary Coded Decimal Interchange Code was developed when the data processing systems had to
communicate with humans using printed characters on a printing device. This code was fine for
communication with humans, but was unsuitable for machine-to-machine communications because
it lacked any form of parity or error-checking capabilities.
BCDIC had the same problem as Baudot – a machine couldn't tell when an error occurred in trans-
mission.
EBCD
Extended Binary-Coded Decimal, developed for the IBM Selectric typewriter, solved the parity
problem. The Selectric principle allowed typing and printing of hard copy, while it could also gener-
ate a unique code suitable for transmitting over a communications facility. Also known in the field
as PTTC (Paper Tape Transmission Code), EBCD uses six information bits and a parity bit so the
receiving system can determine if an error has occurred in a specific character. But EBCD is a se-
quential code with upper and lower case characters. So, while parity could detect errors and pro-
PK232UG Rev. E 9/87
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