Appendix D - Asynchronous Communications
OMG-USB-SER-2
Page
14
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 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 (usually 1, 1.5 or 2). An extra bit used for error detection is
often appended before the stop bits.
Remain Idle or
next start bit
Odd, Even
or
Unused
STOP
P
BIT
5 to 8 Data Bits
Idle state of
line
1
0
1
1.5
2
Figure 4 - 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 refereed 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 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 (i.e. 9600,N,8,1).
