异步串行端口与同步串行端口的不同和用途

onmyway2008

Asynchronous Transmission<br>Asynchronous means &quot;without respect to time.&quot; In terms of data transmission, asynchronous means that no clock or timing source is needed to keep both the sender and the receiver synchronized. Without the benefit of a clock, the sender must signal the start and stop of each character so that the receiver knows when to expect data. <br>Asynchronous transmission is often described as &quot;character-framed&quot; or &quot;start/stop&quot; communication because this method frames each character with a start and stop bit. Each character is typically a 7- or 8-bit value that can represent a number, a letter, a punctuation mark, etc. Each character is preceded by a start bit and followed by a stop bit, or in some cases, two stop bits &#40;see Figure &#41;. An additional bit may be added for parity error checking prior to the first stop bit. <br><br>Synchronous Transmission<br>Synchronous means &quot;with time.&quot; In terms of data transmission, &quot;synchronous&quot; means that a common timing signal is used between hosts. A clock signal is either embedded in the data stream or is sent separately to the interfaces. <br><br>If two hosts use a timing signal to &quot;synch up,&quot; start and stop bits for every 8-bit character value are not necessary. Instead, a large amount of data &#40;e.g., hundreds or even thousands of bytes&#41; can be preceded by synchronization bits. For example, in Ethernet a field of synchronization bits precedes the data payload. This field of synchronization bits, called a preamble, forms a pattern of alternating ones and zeros. The receiver uses this pattern to synchronize with the sender. <br><br>Service providers offer a variety of synchronous and asynchronous WAN services. These services can be grouped into three categories depending on their connection type: <br><br>dedicated connectivity<br>circuit-switched networks<br>packet-switched networks<br><br>