Asynchronous serial communication is a form of serial communication in which
the communicating endpoints' interfaces are not continuously synchronized by a
common clock signal. Instead of a common synchronization signal, the data stream
contains synchronization information in form of start and stop signals, before
and after each unit of transmission, respectively. The start signal prepares
the receiver for arrival of data and the stop signal resets its state to enable
triggering of a new sequence. A common kind of start-stop transmission
is ASCII over RS-232, for example for use in teletypewriter operation.
Origin Mechanical teleprinters using 5-bit
codes typically used a stop period of 1.5 bit times. Very early
electromechanical teletypewriters could demand 2 stop bits to allow mechanical
impression without buffering. Hardware which does not support fractional stop
bits can communicate with a device that uses 1.5 bit times if it is configured
to send 2 stop bits when transmitting and requiring 1 stop bit when receiving.
The format is derived directly from the design of the teletypewriter, which was
designed this way because the electromechanical technology of its day
was not precise enough for synchronous operation: thus the systems needed to be
re-synchronized at the start of each character. Having been re-synchronized,
the technology of the day was good enough to preserve bit-sync for the
remainder of the character. The stop bits gave the system time to recover
before the next start bit. Early teleprinter systems used five data bits,
typically with some variant of the Baudot code.
Very early experimental printing telegraph devices used only a start bit
and required manual adjustment of the receiver mechanism speed to reliably
decode characters. Automatic synchronization was required to keep the
transmitting and receiving units "in step". This was finally achieved by
Howard Krum, who patented the start-stop method of synchronization. Shortly
afterward a practical teleprinter was patented.
Operation Before signalling will work, the sender
and receiver must agree on the signalling parameters:
Full or half-duplex operation The number of bits per character
Endianness: the order in which the bits are sent
The speed or bits per second of the line. Some systems use automatic speed
detection. Whether to use or not use parity
Odd or even parity, if used The number of stop bits sent must be
chosen Mark and space symbols
Asynchronous start-stop signalling was widely used for dial-up modem access to
time-sharing computers and BBS systems. These systems used either seven or eight
data bits. Between computers, the most common
configuration used was "8N1": eight bit characters, with one start bit, one stop
bit, and no parity bit. Thus 10 Baud times are used to send a single
character, which has the nice side-effect of dividing the signalling
bit-rate by ten results in the overall transmission speed in characters per
second. Asynchronous start-stop is the physical
layer used to connect computers to modems for many dial-up Internet access
applications, using a data link framing protocol such as PPP to create packets
made up out of asynchronous serial characters. The performance loss
relative to synchronous access is negligible, as most modern modems will
use a private synchronous protocol to send the data between themselves, and
the asynchronous links at each end are operated faster than this data link,
with flow control being used to throttle the data rate to prevent overrun.
See also Comparison of synchronous and
asynchronous signalling Degree of start-stop distortion
Synchronous serial communication Universal asynchronous
receiver/transmitter References
Further reading Nelson, R. A. and Lovitt, K. M. History
of Teletypewriter Development, Teletype Corporation, retrieved April 14, 2005
Hobbs, Allan G. Five-unit codes, accessed 20 December 2007
External links Programming:Serial Data Communications
at Wikibooks