20
RS-232 SINGLE-PORT PCI CARD
Appendix C. Asynchronous
Communication
Serial data communication 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 communication is the standard means of
serial data communication for PC compatibles and
PS/2
®
computers. The original PC was equipped with a
communication (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 communication.
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.
