串行直连电缆 25针-25针(NullModem)
串行直连电缆用于两台计算机的直接相连，电缆两端的插头都是 25 针母插头：
引脚定义
D-Sub 1
D-Sub 2
Receive Data
3
2
Transmit Data
Transmit Data
2
3
Receive Data
Data Terminal Ready
20
6+8
Data Set Ready + Carrier Detect
System Ground
7
7
System Ground
Data Set Ready + Carrier Detect
6+8
20
Data Terminal Ready
Request to Send
4
5
Clear to Send
Clear to Send
5
4
Request to Send
