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Appendix C: Async 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 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 predefined 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 (usually 1, 1.5, or 2).
•
Idle state of
line
1
0
•
An extra bit used for error detection is often appended before the stop bits.This special bit is called the parity bit.
Parity is the 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 even
parity (E) or odd parity (O). Sometimes parity is not used to detect errors on the data stream. This is referred to as no
parity (N).
Because each bit in asynchronous communications is sent consecutively, it is easy to generalize synchronous
communications by stating that each character is wrapped (framed) by pre-defined bits to mark the beginning and
end of the serial transmission of the character. The data rate and communication parameters for asynchronous
communications have to be the same at both the transmitting and receiving ends. The communication parameters
are baud rate, parity, number of data bits per character, and stop bits (for example, 9600, N, 8, 1).
5 to 8 Data Bits
Figure B-1. Asynchronous Communications Bit Diagram.
Odd, Even
or
Remain Idle or
Unused
next start bit
P
STOP
BIT
1
1.5
2
CHAPTER
15
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