Chapter 1: Installation
Manual Number: 000750-006-5
Page 17
Appendix C
:
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 12: 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 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).
