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Appendix D - Asynchronous Communications
Omega Engineering OMG-COMM8-A Page 16
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 (usual 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 9 - 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 referred 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).