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APPENDIX C: Asynchronous Communications
Appendix C. Asynchronous
Communications
In serial data communication, 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 start 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).
Figure C-1. Asynchronous communications bit diagram.
An extra bit used for error detection is often appended before the stop 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 referred to as (N)o parity. Because each bit in
asynchronous communications is sent consecutively, it is easy to generalize
Odd
Even
or
Unused
Remain Idle
or
Next Start Bit
Parity
Bit
Idle State
of
Line
1
0
5 to 8 Data Bits
1
1.5
2
Stop Bits
