Impulse 2161 - Manual (Page 13)

© Sealevel Systems, Inc. - 11 - SeaLINK+16/232.RJ User Manual
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. A
starting bit followed by a pre-defined number of data bits (5, 6, 7, or 8) defines
character boundaries for asynchronous communications. 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
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).