串行直连电缆 9针-25针(NullModem)
串行直连电缆用于两台计算机的直接相连，电缆两端的插头一头是 9 针母插头，另一头是 25 针母插头：
引脚定义
D-Sub 9
D-Sub 25
Receive Data
2
2
Transmit Data
Transmit Data
3
3
Receive Data
Data Terminal Ready
4
6+8
Data Set Ready + Carrier Detect
System Ground
5
7
System Ground
Data Set Ready + Carrier Detect
6+1
20
Data Terminal Ready
Request to Send
7
5
Clear to Send
Clear to Send
8
4
Request to Send
