Appendix E - Synchronous and Asynchronous Communications
Sealevel Systems
ACB-II
Page
16
Appendix E - Asynchronous and Synchronous
Communications
Serial data communications implies that individual bits of a character are
transmitted consecutively to a receiver that assembles the bits back into a
character. Data rate, error checking, handshaking, and character framing
(start/stop bits or sync characters) are pre-defined and must correspond at
both the transmitting and receiving ends. The techniques used for serial
communications can be divided into two groups,
synchronous
and
asynchronous
.
When contrasting synchronous and asynchronous serial communications,
the fundamental differences deal with how each method defines the
beginning and the end of a character or group of characters. The method of
determining the duration of each bit in the data stream is also an important
difference between synchronous and asynchronous communications. The
remainder of this section is devoted to detailing the differences between
character framing and bit duration implemented in synchronous and
asynchronous communications.
Asynchronous Communications
Asynchronous communications is the standard means of serial data
communication for PC compatibles and PS/2 computers. The original PC
was equipped with a communication or COM: port that was designed
around an 8250
U
niversal
A
synchronous
R
eceiver
T
ransmitter (UART).
This device allows asynchronous serial data to be transferred through a
simple and straightforward programming interface. Character boundaries
for asynchronous communications are defined by a starting bit followed by
a pre-defined number of data bits (5, 6, 7, or 8). The end of the character is
defined by the transmission of a pre-defined number of stop bits (usual 1,
1.5 or 2). An extra bit used for error detection is often appended before the
stop bits.
