© Sealevel Systems, Inc.
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232-DB9 User Manual
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 are the standard means of serial data communication
for PC compatible 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. A starting bit followed by a pre-defined number of data bits (5, 6, 7, or 8)
defines character boundaries for asynchronous communications. 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. The diagram below demonstrates asynchronous communication bits.
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).
Remain Idle or
next start bit
Unused
STOP
P
BIT
5 to 8 Data Bits
1
0
1
1.5
2
Odd, Even
or
Idle state of
line
