PC Hardware A Beginner’s Guide

These bits mark the beginning and ending of each character transmitted. The start bit has

a value of 0, and the stop bit is set to a 1.

If parity is in use, a parity bit is added to the data block to help ensure that the data

sent is what arrives. Parity forces the count of 1 bits in the transmitted character to either

an even or an odd number. For example, if an uppercaseAis transmitted, the binary for-

mat of 01000001 is what is actually transmitted. If even parity is in use, the parity bit that

isaddedtotheendissetto0becauseanevennumberof1bitsarepresentinthecharacter.

Ifoddparityisinuse,theparitybitissettoa1toforceanoddnumberof1bitsinthechar-

acter. If the receiving device detects the wrong number of 1 bits in a character in comparison

to its parity method, it requests the sending device to resend the character.

So,witheverythingaddedtotheASCIIbinarycharacter,thedatablockendsupbeing

11 bits long, as follows:

Transmitted character:A

Start bit: 0

ASCII binary data pattern: 01000001

Even-parity bit: 0

Stop bit: 1

Transmitted data block: 00100000101

The UART

A universal asynchronous receiver/transmitter (UART, pronounced “you-art”) controls

serial ports and devices. This specialized integrated circuit is found either on the device

adapter card or on the motherboard. The UART chip controls all actions and functions of

the serial port, including:

 Controlling all the connectors’ pins and their associated signals

 Establishing the communication protocol

 Converting the parallel format bits of the PC’s data bus into a serial bit stream

for transmission

(^486) PC Hardware: A Beginner’s Guide
Figure 19-6. Asynchronous communications send data in five- to eight-byte blocks separated by
idle periods of varying sizes