©Sealevel Systems, Inc.
7404e Manual
22
SL9224 02/2017
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. 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.
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 5 - 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).
