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USER'S GUIDE
ASYNCHRONOUS VERSUS SYNCHRONOUS TRANSMISSION
APPENDIX F – ASYNCHRONOUS VERSUS SYNCHRONOUS TRANSMISSION
Since the beginning of electromechanical telegraphy and teleprinting, the problem of maintaining
the proper timing relationship between the sender and the receiver has been resolved by mechani-
cal synchronization methods. These were based on the basic ideas of 'start-stop' telegraphy.
The original Baudot rotary distributor permitted the sending distributor to tell the receiving distrib-
utor when the rotary contact was at a reference point in the rotation of the motor shaft. The dis-
tributor created a start and stop pulse at the beginning and end of each character. In teleprinter
operation, each character has carried its own sync information in the form of the traditional 'start'
element at the beginning of each character, and the 'stop' bit at the end of the character.
In some systems, the 'stop' element is one, one-and-one-half, or two bits. The result is reduced
efficiency - each character has a fixed amount of 'overhead.' These housekeeping bits carry no
user information. They exist only to keep the sending and receiving machines in sync. This re-
duced efficiency has kept a lid on higher data rates and has been a block in the constant search
for faster ways of sending more information for more people.
As data processing requirements became more demanding newer forms of synchronization
evolved. A method was developed which applied a single synchronizing string to an entire string of
characters, rather than include sync information in each character. The 'overhead', or housekeep-
ing bits previously inserted into each character for sync purposes were eliminated; higher data
rates became practical without sacrificing data integrity. This newer method, called 'synchronous'
transmission, is used in higher speed and specialized forms of transmission.
Unlike conventional Baudot and ASCII RTTY operation, both AMTOR modes, ARQ and FEC, use
synchronous, rather than asynchronous transmission.
Your computer keyboard sends asynchronous data to your PK-232 with start and stop bits in each
character.
When you enter the AMTOR operating modes, the PK-232 program routines strip the start and
stop bits from the characters and establish the synchronous transmission required in AMTOR.
The PK-232 has a versatile software program for digital communications techniques that gives you
computer-based Morse code, and the choice of four different data transmission codes: Baudot, AS-
CII, AMTOR, and Packet-Radio.
The question of transmission speed is discussed separately. It should be understood that the
speed at which a digital code is used has no real relationship to the choice of the code itself. As a
general rule, any code can be used at any speed within the capacity of the system software.
PK232UG Rev. E 9/87
F-1
188
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