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.

You probably already saw this connector on the back of your PC.

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.

• 一个简短的关于异步通信的页面
• 一个关于RS232串行通信的介绍