## RS-232串行接口

An RS-232 interface has the following characteristics:

  * Uses a 9 pins connector "DB-9" (older PCs use 25 pins "DB-25").
  * Allows bidirectional full-duplex communication (the PC can send and receive data at the same time).
  * Can communicate at a maximum speed of roughly 10KBytes/s.

### DB-9连接器

You probably already saw this connector on the back of your PC.
{{ :serialconnector.jpg |}}

It has 9 pins, but the 3 important ones are:
  * pin 2: RxD (receive data).
  * pin 3: TxD (transmit data).
  * pin 5: GND (ground).

Using just 3 wires, you can send and receive data.

Data is commonly sent by chunks of 8 bits (we call that a byte) and is "serialized": the LSB (data bit 0) is sent first, then bit 1, ... and the MSB (bit 7) last.


### 资源链接
  * 一个简短的[[http://penguin.dcs.bbk.ac.uk/academic/technology/physical-layer/asynchronous/|关于异步通信的页面]]
  * 一个[[http://www.taltech.com/datacollection/articles/serial_intro|关于RS232串行通信的介绍]]