Asynchronous Serial Communications Programmer's Reference Manual (32022-90052)
22 Chapter1
Introduction
Asynchronous Serial Communications
Asynchronous Serial Communications
Asynchronous Serial Communications (ASC) is the term used to
describe the speciļ¬c manner in which communications take place
between an MPE/iX computer and its associated terminals and serial
printers.
Asynchronous refers to the scheme used to let the receiving end of a
communication link know when data is being sent and when
transmission of the data has completed. Each character is preceded by
a special bit, called a start bit, which signals its arrival. The character
is then followed by at least one stop bit, a second special bit which
signals that transmission of that character is complete. Because this
scheme allows data to be sent intermittently and does not require
elaborate timing mechanisms, the asynchronous method is particularly
useful for reading characters entered through a keyboard, where the
time interval between the entry of two characters can vary greatly.
Serial transmission refers to the way each character is transferred.
Characters transmitted serially are sent one bit at a time and received
one bit at a time in the order of transmission. This is in contrast to
parallel transmission schemes which transmit more than one bit at a
time.
Figure 1-1 depicts transmission of data characters in an asynchronous
serial mode. The arrow indicates the direction of the transmission.
Figure 1-1 Asynchronous Serial Transmission
Devices
The devices which communicate with MPE/iX asynchronously include a
number of supported terminals and serial printers. Modems may also
be a part of the communications link. The characteristics of each device
are determined by the MPE/iX operating system as well as by the
device itself.
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