Manual Number: 00750-903-2
Page 16
WINCOMM8/C Manual
Appendix C- 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 com-
patibles 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.
5HPDLQ,GOHRU
QH[WVWDUWELW
2GG(YHQ
RU
8QXVHG
6723
3
%,7
WR'DWD%LWV
,GOHVWDWHRI
OLQH
Figure 8: 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. Some-
times 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).
