Asynchronous start-stop
Encyclopedia
Asynchronous serial communication describes an asynchronous
, serial transmission protocol
in which a start signal is sent prior to each byte, character or code word and a stop signal is sent after each code word. The start signal serves to prepare the receiving mechanism for the reception and registration of a symbol and the stop signal
serves to bring the receiving mechanism to rest in preparation for the reception of the next symbol. A common kind of start-stop transmission is ASCII
over RS-232
, for example for use in teletypewriter operation.
In the diagram, two byte
s are sent, each consisting of a start bit, followed by eight data bits (bits 0-7), and one stop bit, for a 10-bit character frame. The last data bit is sometimes used as a parity bit. The number of data and formatting bits, the order of data bits
, the presence or absence of a parity bit, the form of parity (even or odd) and the transmission speed must be pre-agreed by the communicating parties.
The "stop bit" is actually a "stop period"; the stop period of the transmitter may be arbitrarily long. It cannot be shorter than a specified amount, usually 1 to 2 bit times. The receiver requires a shorter stop period than the transmitter. At the end of each character, the receiver stops briefly to wait for the next start bit. It is this difference which keeps the transmitter and receiver synchronized.
s using 5-bit codes (see Baudot code
) typically used
a stop period of 1.5 bit times.
Very early electromechanical teletypewriters (pre-1930) 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, (an electrical engineer and son of Charles Krum
) who patented the start-stop method of synchronization , granted September 19, 1916 then , granted December 3, 1918. Shortly afterward a practical teleprinter
was patented July 3, 1917.
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 stop bit and no parity bit. Thus 10 Baud times are used to send a single character, which has the nice side-effect that 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 comparison of synchronous and asynchronous signalling
for alternatives to asynchronous start/stop operation.
Asynchronous communication
In telecommunications, asynchronous communication is transmission of data without the use of an external clock signal, where data can be transmitted intermittently rather than in a steady stream. Any timing required to recover data from the communication symbols is encoded within the symbols...
, serial transmission protocol
Serial communication
In telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels...
in which a start signal is sent prior to each byte, character or code word and a stop signal is sent after each code word. The start signal serves to prepare the receiving mechanism for the reception and registration of a symbol and the stop signal
Stop signal
In telecommunication, the term stop signal has the following meanings:1. In asynchronous serial communication, a signal at the end of a character that prepares the receiving device for the reception of a subsequent character...
serves to bring the receiving mechanism to rest in preparation for the reception of the next symbol. A common kind of start-stop transmission is ASCII
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
over RS-232
RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
, for example for use in teletypewriter operation.
In the diagram, two byte
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
s are sent, each consisting of a start bit, followed by eight data bits (bits 0-7), and one stop bit, for a 10-bit character frame. The last data bit is sometimes used as a parity bit. The number of data and formatting bits, the order of data bits
Endianness
In computing, the term endian or endianness refers to the ordering of individually addressable sub-components within the representation of a larger data item as stored in external memory . Each sub-component in the representation has a unique degree of significance, like the place value of digits...
, the presence or absence of a parity bit, the form of parity (even or odd) and the transmission speed must be pre-agreed by the communicating parties.
The "stop bit" is actually a "stop period"; the stop period of the transmitter may be arbitrarily long. It cannot be shorter than a specified amount, usually 1 to 2 bit times. The receiver requires a shorter stop period than the transmitter. At the end of each character, the receiver stops briefly to wait for the next start bit. It is this difference which keeps the transmitter and receiver synchronized.
Origins with teletypewriters
Mechanical teleprinterTeleprinter
A teleprinter is a electromechanical typewriter that can be used to communicate typed messages from point to point and point to multipoint over a variety of communication channels that range from a simple electrical connection, such as a pair of wires, to the use of radio and microwave as the...
s using 5-bit codes (see Baudot code
Baudot code
The Baudot code, invented by Émile Baudot, is a character set predating EBCDIC and ASCII. It was the predecessor to the International Telegraph Alphabet No 2 , the teleprinter code in use until the advent of ASCII. Each character in the alphabet is represented by a series of bits, sent over a...
) typically used
a stop period of 1.5 bit times.
Very early electromechanical teletypewriters (pre-1930) 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
Comparison of synchronous and asynchronous signalling
Synchronous and asynchronous transmissions are two different methods of transmission synchronization. Synchronous transmissions are synchronized by an external clock, while asynchronous transmissions are synchronized by special signals along the transmission medium.-The need for...
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
Baudot code
The Baudot code, invented by Émile Baudot, is a character set predating EBCDIC and ASCII. It was the predecessor to the International Telegraph Alphabet No 2 , the teleprinter code in use until the advent of ASCII. Each character in the alphabet is represented by a series of bits, sent over a...
.
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, (an electrical engineer and son of Charles Krum
Charles Krum
Charles L. Krum was a key figure in the development of the teleprinter, a machine which played a key role in the history of telegraphy and computing....
) who patented the start-stop method of synchronization , granted September 19, 1916 then , granted December 3, 1918. Shortly afterward a practical teleprinter
Teleprinter
A teleprinter is a electromechanical typewriter that can be used to communicate typed messages from point to point and point to multipoint over a variety of communication channels that range from a simple electrical connection, such as a pair of wires, to the use of radio and microwave as the...
was patented July 3, 1917.
Asynchronous start/stop operation
Before signalling will work, the sender and receiver must agree on the signalling parameters:- full or half-duplexDuplex (telecommunications)A duplex communication system is a system composed of two connected parties or devices that can communicate with one another in both directions. The term multiplexing is used when describing communication between more than two parties or devices....
operation - the number of bits per character
- endiannessEndiannessIn computing, the term endian or endianness refers to the ordering of individually addressable sub-components within the representation of a larger data item as stored in external memory . Each sub-component in the representation has a unique degree of significance, like the place value of digits...
– the order in which the bits are sent - the speed or bits per second of the line (often incorrectly referred to as the BaudBaudIn telecommunications and electronics, baud is synonymous to symbols per second or pulses per second. It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes made to the transmission medium per second in a digitally modulated signal or a...
rate). Some systems use automatic speed detection. - both sides must agree to use or not use parity
- if parity is used, both sides must agree on using odd or even parity
- the number of stop bits sent must be chosen (the number sent must be at least what the receiver needs)
- Mark and space symbols (current directions in early telegraphy, later voltage polarities in EIAEIAEIA may refer to:*Edmonton International Airport*EIA Multiport, a North American NTSC SCART connector*Electronic Industries Alliance , a US trade organization...
RS-232RS-232In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
etc., frequency shift polarities in frequency shift keying, etc.)
Asynchronous start-stop signalling was widely used for dial-up modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
access to time-sharing
Time-sharing
Time-sharing is the sharing of a computing resource among many users by means of multiprogramming and multi-tasking. Its introduction in the 1960s, and emergence as the prominent model of computing in the 1970s, represents a major technological shift in the history of computing.By allowing a large...
computers and BBS
Bulletin board system
A Bulletin Board System, or BBS, is a computer system running software that allows users to connect and log in to the system using a terminal program. Once logged in, a user can perform functions such as uploading and downloading software and data, reading news and bulletins, and exchanging...
systems. These systems used either seven or eight data bits.
Between computers, the most common configuration used was "8N1
8-N-1
8-N-1 is a common shorthand notation for a serial port parameter setting or configuration in asynchronous mode, in which there are eight data bits, no parity bit, and one stop bit....
": eight bit characters, with one stop bit and no parity bit. Thus 10 Baud times are used to send a single character, which has the nice side-effect that dividing the signalling bit-rate by ten results in the overall transmission speed in characters per second.
Asynchronous start-stop is the physical layer
Physical layer
The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
used to connect computers to modems for many dial-up Internet access applications, using a data link framing protocol such as PPP
Point-to-Point Protocol
In networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
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
Flow control
In data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with...
being used to throttle the data rate to prevent overrun.
See comparison of synchronous and asynchronous signalling
Comparison of synchronous and asynchronous signalling
Synchronous and asynchronous transmissions are two different methods of transmission synchronization. Synchronous transmissions are synchronized by an external clock, while asynchronous transmissions are synchronized by special signals along the transmission medium.-The need for...
for alternatives to asynchronous start/stop operation.
See also
- Universal asynchronous receiver/transmitterUniversal asynchronous receiver/transmitterA universal asynchronous receiver/transmitter, abbreviated UART , is a type of "asynchronous receiver/transmitter", a piece of computer hardware that translates data between parallel and serial forms. UARTs are commonly used in conjunction with communication standards such as EIA RS-232, RS-422 or...
- ModemModemA modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
- Degree of start-stop distortionDegree of start-stop distortionIn telecommunication, the term degree of start-stop distortion has the following meanings:# In asynchronous data transmission, the ratio of the absolute value of the maximum measured difference between the actual and theoretical intervals separating any significant instant of modulation from the...
- Serial communicationSerial communicationIn telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels...
- Synchronous serial communication
Further reading
- Nelson, R. A. and Lovitt, K. M. History of Teletypewriter Development (October 1963), Teletype Corporation, retrieved April 14, 2005
- Hobbs, Allan G. (1999) Five-unit codes, accessed 20 December 2007