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APPENDIX C: Asynchronous Communication
Appendix C: Asynchronous
Communication
In serial data communication, 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 communication is the standard means of serial data
communication for PC and compatible computers. The original PC was equipped
with a communication or COM port that was designed around an 8250 Universal
Asynchronous Receiver Transmitter (UART), which allows asynchronous serial
data to be transferred through a simple and straightforward programming device.
The Serial Host Adapter (IC142C) uses a much newer 16554 UART that also
features buffering.
Asynchronous communication works this way: A start bit, followed by a pre-
defined number of data bits (5, 6, 7, or 8) defines character boundaries for
asynchronous communication. The end of the character is defined by the
transmission of a pre-defined number of stop bits (usually 1, 1.5, or 2).
Figure C-1. Asynchronous Communication Bit Diagram.
Odd
Even
or
Unused
Remain Idle
or
Next Start Bit
Parity
Bit
Idle State
of
Line
1
0
5 to 8 Data Bits
1
1.5
2
Stop Bits