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2-PORT RS-232/422/485 PCI HOST ADAPTER
Appendix D:
Asynchronous Communication
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 D-1. Bit diagram for asynchronous communication.
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 refereed to as (N)o parity.
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