串行直连电缆 9针-25针(NullModem)
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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 |