Impulse SIO-104+2 (3502) User Manual download pdf (Page 15)

Appendix C - Electrical Interface

Sealevel Systems SIO-104+2 Page 13

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

Figure 3 - 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 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).

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Impulse SIO-104+2 (3502) User Manual Page 15