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RS-422/485 SERIAL INTERFACE PLUS, 4-PORT
Appendix D: Asynchronous
Communications
In serial data communications, 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. 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 (usual
1, 1.5 or 2).
Remain idle or
next start bit
Odd, Even
or
Unused
Idle state of
line
5 to 8 Data Bits
1.5
1
2
STOP
P
BIT
1
0
Figure D-1. Asynchronous Communications Bit Diagram.