User guide

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SeaLINK+16.VC 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 o

next start bit

Odd, Even

Unused

STOP

BIT

5 to 8 Data Bits

Idle state of

line

1.5

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).