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Table Of Contents  CertiGuide to A+ (Core Hardware)
 9  Chapter 0111:  Peripheral Devices
      9  III  Peripheral Device Interfaces
           9  Serial

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Asynchronous Versus Synchronous

The two protocols used for serial communications with PCs are synchronous and asynchronous communications. It is a common misnomer among many PC techs is that synchronous communications are full duplex and asynchronous communications are half duplex. This is not the case.

These two communication methods dictate how devices talk to each other and whether or not they are synchronized with each other. In synchronous communications, before the data is sent, the two devices will synchronize with each other. When communications are idle, the two devices send special characters to each other to keep themselves in synch.

In asynchronous communications, the two devices that are communicating are not synchronized. Therefore, they have to tell each other when they're starting a data transmission and when they are ending a data transmission.

This is done through the use of start and stop bits. A start bit has a value of zero. An idle bit has a value one. So when an idle device begins a transmission, the receiving device will sense that the line has switched from one to zero, thus alerting the receiving device the data is about to come down the line. When the data is done being transmitted, a stop bit is sent, having a value of one. Serial communications can be configured to send five, six, seven, or eight data bits between each start and stop bit.

Both the receiving and transmitting devices must, however, agree on the number of data bits and the speed data will be sent and received. Most PC devices use either seven or eight data bits.

Most modern PC's use asynchronous mode for serial communications. A special chip called a Universal Asynchronous Receiver Transmitter, or UART chip controls these communications. Most modern PCs come equipped with a 16550 or 16450 UART chip. Of the two, the 16550 is faster, allowing transmission speeds up to 153,000 bps.

[spacer]Bits vs. Baud

Some of the most confusing elements of serial communications are related to the terms bits per second (bps) and baud (pronounced bawd). The term baud comes from J.M.E. Baudot who invented the Baudot telegraph code.

Baud refers to the number of signaling elements that occur each second. Being that serial communications send 1 bit at a time, baud is the number of bits per second the line can carry. So, 1200 baud means that 1200 bits per second are being transmitted.

It is a fact that the FCC has limited the phone lines in the United States to 2400 baud.

So then how do we use modems that transmit 56,000 bps?

The answer lies in the fact that technology has developed which allows us to pack more bits in a single signaling element.

This is achieved through a combination of data compression and phase modulation of the signals. First the data is compressed using an industry standard algorithm. Once it is compressed, the signals are modulated to allow for multiple signals to be sent in one cycle.



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