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Asynchronous and Synchronous
Transmission
Words and word combinations:
Patterns of bits – система битов
one character – один значок
asynchronous transmission – асинхронная, не совпадающая по времени передача
start-stop transmission – прерывистая передача
bracketed by control bits – группируется контрольными битами
wrapped – заворачивается
envelope – конверт
error check bit – проверяющий ошибки бит
a parity scheme – аналогичная, соответствующая схема
load the communications software – загрузка программ связи
agreed-on parity scheme – соглашение на соответствующую схему
one- odd or even – один четный или нечетный
synchronous transmission – синхронная, одновременная передача
header and trailer bites – головные и хвостовые биты
huge volumes of data – данные огромного объема
expensive and complex timing devices – дорогие и сложные приборы времени
When signals are transmitted through modems from one computer to another, patterns of bits coded to represent data are sent one bit at a time. How does the receiving device know where one character ends and another starts? In asynchronous transmission, also called start-stop transmission, each string of bits that make up a character is bracketed by control bits. In effect, each group of digital or analog signals making up the code for one character is individually ‘wrapped’ in an electronic ‘envelope’ made of a start bit, one or two stop bits, and error check bit (or parity bit). The error check bit is set according to a parity scheme that can be odd or even.
Users of both sending and the receiving computers must agree on the parity scheme. The scheme chosen is set within a communications software program. For example, if computer Ais transmitting to computer B, the users of both computers must first agree on a parity scheme (perhaps by having a telephone conversation), then load the communications software, and choose the agreed-on parity scheme by choosing the appropriate software option. It doesn’t matter which scheme is chosen as long as both computers are using the same one- odd or even.
Because asynchronous communication is inexpensive, it is widely used; however, it is also relatively slow, because of the number of parity and error check bits that must be transmitted with the data bits.
Data
communication
link
Analog
Transmission
In synchronous transmission, characters can be sent much faster because they are sent as blocks. Header and trailer bytes, called flags, are inserted as identifiers at the beginnings and the ends of blocks. Large computers to transmit huge volumes of data at high speeds use synchronous transmission. Expensive and complex timing devices must be used to keep the transmission activities synchronized.
Most microcomputers are not equipped to handle synchronous data transmission. However, users recognize that synchronous transmission allows them to quickly download files from the company’s main computer system into their microcomputers and to upload data that has been initially captured on these microcomputers to the company’s main computer system, thus avoiding duplication of data entry on both systems. As a result, a number of vendors have produced add-on boards.