Appendix D - Asynchronous Communications
Sealevel Systems
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Appendix D - Asynchronous 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) are pre-defined and must correspond at both the transmitting
and receiving ends.
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 Universal Asynchronous Receiver Transmitter (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 (usually 1, 1.5 or 2). An extra
bit used for error detection is often appended before the stop bits.
Remain Idle or
next start bit
Odd, Even
or
Unused
STOP
P
BIT
5 to 8 Data Bits
Idle state of
line
1
0
1
1.5
2
Figure 1 - Asynchronous Communications Bit Diagram
This special bit is called the parity bit. Parity is a simple method of determining
if a data bit has been lost or corrupted during transmission. There are several
methods for implementing a parity check to guard against data corruption.
Common methods are called (E)ven Parity or (O)dd Parity. Sometimes parity
is not used to detect errors on the data stream. This is refereed to as (N)o
parity. Because each bit in asynchronous communications is sent
consecutively, it is easy to generalize asynchronous communications by stating
that each character is wrapped (framed) by pre-defined bits to mark the
beginning and end of the serial transmission of the character. The data rate and
communication parameters for asynchronous communications have to be the
same at both the transmitting and receiving ends. The communication
parameters are baud rate, parity, number of data bits per character, and stop bits
(i.e. 9600,N,8,1).
