AT控制命令，MODEM连线标准

  l         MODEM标准电缆联接：

MODEM端25PIN(公) 终端端9PIN(母)
8(CD)        1(CD Carrier Detect)
3(RD)        2(RD Receive Data)
2(TD)        3(TD Transmit Data)
20(DTR)     4(DTR Data Terminate Ready)
7(GND)      5(SG Singal Ground)
6(DSR)       6(DSR Data Set Ready)
4(RTS)       7(RTS Request To Send)
5(CDS)       8(CTS Clear To Send)
22(RI)        9(RI Ring Indicate)   

三线电缆方式：
MODEM端25PIN(公) 终端端9PIN(母)
3(RD)        2(RD Receive Data)
2(TD)        3(TD Transmit Data)