Appendix D - Asynchronous Communications
Appendix D - Asynchronous Communications
Serial data communications 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
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
1.5or 2). An extra bit used for error detection is often appended before the stop bits.
Idle state of |
| Odd, Even |
|
line | 5 to 8 Data Bits | or | Remain Idle or |
| Unused | next start bit | |
|
|
1
0
P 
STOP
BIT
1
1.5
2 
Figure 9 - Asynchronous Communications Bit Diagram
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 asynchronous communications by stating that each character is wrapped (framed) by
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