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
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
1, 1.5 or 2).
Idle state of
line
1
0
Odd, Even |
|
or | Remain idle or |
Unused | next start bit |
5 to 8 Data Bits
P STOP
BIT
1 
1.5 
2
Figure D-1. Asynchronous Communications Bit Diagram.
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