Manual

Appendix E - Asynchronous and Synchronous Communications
Sealevel Systems ACB-104 Page 17
Appendix E - Asynchronous and Synchronous 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 or sync characters) are pre-defined and must correspond at both the transmitting and receiving ends. The
techniques used for serial communications can be divided two groups, asynchronous and synchronous.
When contrasting asynchronous and synchronous serial communications, the fundamental differences deal with
how each method defines the beginning and end of a character or group of characters. The method of determining
the duration of each bit in the data stream is also an important difference between asynchronous and synchronous
communications. The remainder of this section is devoted to detailing the differences between character framing
and bit duration implemented in asynchronous and synchronous communications.
Asynchronous Communications
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 (usual 1, 1.5 or 2). An extra bit
used for error detection is often appended before the stop bits.
1
1.5
2
Stop Bits
Parity
Bit
5 to 8 Data Bits
Odd
Even
or
Unused
Remain Idle
or
Next Start Bit
1
0
Idle State
of
Line
Figure 8 - 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).