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.
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